fix(cli): no-TTY fallback for task pickers

[olddognewflex]

Closes #62.

Problem

qi task done / schedule / breakdown invoked the Bubble Tea picker on an ambiguous match with no TTY guard. In a pipe (or under an agent), that dies with a raw bubbletea open /dev/tty error. qi inbox has --dry-run as its headless path; the pickers had nothing (§2.4).

Fix

The three picker calls now route through a pickTasks wrapper:

• With a terminal — launches the interactive picker, unchanged.
• Without one — prints the candidate list plus an instruction ("Re-run with a more specific query (or an exact single match)…") and returns a clear ambiguous match: N tasks and no terminal for the picker error, exiting non-zero.

TTY detection is stdinIsTTY, a package var over term.IsTerminal(os.Stdin) so tests can force the headless path.

Verification

$ qi task done write </dev/null
Tasks matching "write" — 2 candidates; the interactive picker needs a terminal.
  - write report
  - write tests
Re-run with a more specific query (or an exact single match) to select non-interactively.
ambiguous match: 2 tasks and no terminal for the picker   # exit 1

Test forces the no-TTY branch and asserts the candidate list + instruction + error. Full go test ./..., go vet, go build ./... green.

Merge-order note

Touches internal/commands/task.go. Overlaps #60 (constructor swap) and #61; a trivial rebase for whichever merges later.