feat(ui): add huh-backed prompts + detector.ResolveInteractive

[dipto0321]

Summary

Adds huh-backed interactive prompts (internal/ui/prompt.go) and detector.ResolveInteractive to handle the "multiple Node version managers detected, no preference given" case. The post-upgrade cleanup decision point is migrated from raw stdin line-reads (promptAllOrNothing / promptPerVersion) to the new ui.Prompt abstraction so FancyMode gets huh's arrow-key navigation and PlainMode gets a numbered-line fallback.

This is PR3 of 3 for #105 (PR1 was the scaffold via #74; PR2 was #117 spinners via bubbletea; PR4 will follow).

What this PR delivers

internal/ui/prompt.go (new)

• Prompt interface: Confirm(question, defaultYes), Select(prompt, options, defaultLabel), Mode().
• NewPrompt(mode, in, out) — FancyMode + non-nil streams → fancyPrompt (huh.Form per call); otherwise plainPrompt.
• plainPrompt reads one line per question. EOF / empty input fall back to the default (Confirm → defaultYes; Select → defaultLabel / options[0]). I/O errors propagate.
• fancyPrompt builds a fresh huh.Form per call (no cached state) and pipes through huh.ErrUserAborted → "use the default" mapping.
• prompt_test.go covers Confirm y/n/Y/yes/empty/EOF/case-insensitive/variants, Select numeric/label/empty/EOF/no-options, bufio.Reader passthrough, nil-stream degradation under FancyMode, and I/O error surfacing. All tests pass under -race.

internal/detector/interactive.go (new)

• ResolveManagerAuto(reg, preferred) — wrapper around ResolveManager that returns a new ErrInteractiveRequired sentinel when multiple managers are detected and no preference is set. Existing ResolveManager semantics preserved.
• ResolveInteractive(reg, prompt, nonInteractive, in, out) — for the multi-manager case. Single-manager registries short-circuit (no prompt). nonInteractive=true returns ErrInteractiveRequired immediately (the --yes / CI path) so the caller can surface "use --manager". Order is stable via Priority.
• interactive_test.go covers zero/single/multi/no-pref paths, the fallback plain-prompt against supplied in/out streams (EOF → first option), the nonInteractive shortcut, and a stubPrompt for future test mocks.

internal/cli/cleanup.go (rewritten prompts)

• runCleanupPrompt(prompt, writer, cfg, toInstall, installed, active, m) — new signature threads a ui.Prompt + ui.Writer through. Plain-mode tests pass a ui.NewPrompt(PlainMode, ...) from internal/ui; production code calls ui.NewPrompt(writer.Mode(), cmd.InOrStdin(), cmd.OutOrStdout()) once per upgrade.
• promptAllOrNothing now constructs a Select with two top-level options ("Delete all of the above", "Skip cleanup") plus one "Delete vX.Y.Z" entry per candidate. The "delete one specific version" path is now a labeled option rather than the old "type a version string" pattern — same UX, more discoverable.
• promptPerVersion uses ui.Prompt.Confirm with defaultYes=false. Empty input defaults to "no" — the safe direction for cleanup prompts.
• All progress lines ("No old versions to clean up", "Cleanup skipped", "Deleted vX", "Failed to delete") route through ui.Writer.{Info,Success,Warn} so Plain-mode gets unstyled text and Fancy-mode gets colored prefixes (✓/⚠/•).
• The cleanupIO struct (was: bufio.Reader + io.Writer) is gone — that's the migrated surface.

internal/cli/cleanup_test.go (rewired)

• newCleanupIO (cleaned up; replaced by newTestPrompt and promptWithInput).
• The text-input tests ("y\n", "20.18.0\n") become numeric-input tests ("1\n" for "Delete all", "2\n" for "Skip cleanup", "3\n"–"5\n" for per-version picks). This is a deliberate UX improvement: the new prompt asks a multiple-choice question with a labeled option list, not a free-form text field.
• All existing regression coverage retained: #76 "sticky-up after y at all-or-nothing", #58 ForcePerVersion downgrade, excludes-new-and-active, no-candidates, prefiltered-only, uninstall-error-collected.

internal/cli/upgrade.go

• detector.ResolveManager → detector.ResolveManagerAuto + detector.ResolveInteractive handoff. The ErrInteractiveRequired branch hands off with yes mapped to nonInteractive so a CI script with -y but no --manager errors out with the "use --manager" hint instead of hanging on stdin.
• runCleanupPrompt call site now constructs both prompt and writer once and threads them through.

go.mod / go.sum

• Adds github.com/charmbracelet/huh v1.0.0 + transitive deps (charmbracelet/bubbles, charmbracelet/x/exp/strings, dustin/go-humanize, mitchellh/hashstructure/v2, atotto/clipboard, catppuccin/go, mattn/go-localereader was already a bubbletea transitive, plus a few more — see go.sum).

Edge cases I considered before opening

• EOF / empty input on Confirm: Plain prompt returns defaultYes so a piped echo "" | nodeup upgrade without -y doesn't accidentally mass-delete. Fancy prompt pre-fills the huh toggle with the default. prompt_test.go pins both paths.
• EOF / empty input on Select: Plain prompt returns defaultLabel or options[0]. Fancy prompt's abort handler returns the default. Closed-stdin CI scripts still pick something — the safest available — and don't hang.
• Single manager detected: ResolveInteractive short-circuits in len(reg.Found) == 1 before any prompt call. No spinner, no huh build, no cost.
• Zero managers detected: returns ErrNoManager (existing sentinel). Caller already handles this on the upgrade path.
• Multi-manager + nonInteractive: returns ErrInteractiveRequired without reading stdin. The UI layer can surface "use --manager " cleanly. interactive_test.go pins this.
• Multi-manager + interactive + EOF: falls back to options[0] per the Prompt.Select contract. User probably expected "fnm" (top-priority manager) anyway — matches the platform's Priority() order.
• Multi-manager + interactive + garbage answer: Plain prompt falls back to default after an unrecognized answer; Fancy prompt gives huh one re-prompt cycle. Same UX as Confirm.
• What about huh form lifecycle: Each fancyPrompt.{Confirm,Select} builds a fresh huh.Form and runs .Run() to completion. No global state, no caching, no signal-handler installation (huh's default SIGINT handler is left in place because we trust the bubbletea stream handler we set in PR2 to never race against plain prompt paths).
• Prompt printed in cleanup + Writer printed in upgrade.go: Both print to the same sink (cmd.OutOrStdout() for the prompt line, separate for the writer). Different files = no buffered-output race; tests confirm with bytes.Buffer shared between both.
• Empty options to Select: Returns an error from both Plain and Fancy paths. Defensive — callers always pre-validate.
• The #76 regression test under the new prompt flow: A user who selects "Delete all" (option index 1) no longer gets per-version [y/N] re-prompts. The sticky-up logic still lives in runCleanupPrompt Step 4 (cfg.PerVersion = false unless ForcePerVersion). The test for this is TestCleanupPrompt_DeleteAllSkipsPerVersionPrompt and was updated to check for the absence of "[y/N]" in the output rather than the old "Delete v" string (which now legitimately appears in the Select option list and isn't the regression marker).

Verification

$ make ci
go mod tidy
gofmt -s -w .
go vet ./...
golangci-lint run ./...
0 issues.
go test -race -coverprofile=coverage.out -covermode=atomic ./...
ok  	github.com/dipto0321/nodeup/internal/cli	coverage: 33.3%
ok  	github.com/dipto0321/nodeup/internal/detector	coverage: 80.2%
ok  	github.com/dipto0321/nodeup/internal/ui	coverage: 67.4%
total: 62.4%

Plus a manual make build && ./bin/nodeup upgrade --help confirming the binary still launches cleanly.

Out of scope (deferred to PR4 per #105 phasing)

• Migrating cmd.Printf calls in check.go, list.go, packages.go, config.go, and the remaining upgrade.go lines to ui.Writer.{Info,Success,Warn,Error,Println}. PR4 is purely mechanical at this point: the Prompt and Writer surfaces are stable, so PR4 doesn't need to revisit this PR's code, just swap call sites in adjacent files.
• A new file internal/ui/report.go for the end-of-upgrade summary block (some of those lines go through ui.Writer together).
• After PR4, SPEC V2 flips from aspirational to enforced (every user-facing string in internal/cli/* flows through internal/ui).

Part of #105. PR4 to follow in this same issue.

[cocogitto-bot]

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