feat(cli): add deepcode login command to save API key

[dongowu]

Summary

Adds a deepcode login sub-command that interactively saves the DeepSeek API key to ~/.deepcode/settings.json, writing a ready-to-use default config so a fresh login works out of the box:

{
  "env": {
    "MODEL": "deepseek-v4-pro",
    "BASE_URL": "https://api.deepseek.com",
    "API_KEY": "sk-..."
  },
  "thinkingEnabled": true,
  "reasoningEffort": "max"
}

When updating an existing settings file, custom fields (model, base URL, thinking, permissions, …) are preserved — only env.API_KEY is overwritten and missing defaults are filled in.

Usage

deepcode login                     # hidden input (Backspace/Ctrl+U/Ctrl+C aware)
deepcode login --show              # echo the key while typing
deepcode login --api-key sk-xxx    # non-interactive (CI/scripts); -k alias
echo sk-xxx | deepcode login       # piped stdin (non-TTY fallback)
deepcode login --help              # login help

Design

• Dispatched before the TUI: in main(), login is handled after --version/--help but before the interactive-terminal check and Windows shell config, so it runs without a TTY (keys can be piped) and never starts Ink.
• Isolated from the existing yargs parser: a dedicated parseLoginArgs() short-circuits ahead of yargs when argv[0] === "login". The strict TUI parser is untouched, so normal usage (deepcode, -p, -r, positional query) is unaffected — covered by regression tests.
• core: exports buildLoginSettings() to merge existing settings with the default template + new API key (reuses DEFAULT_MODEL / DEFAULT_BASE_URL).
• Hidden input uses raw mode on a TTY; falls back to a single-line stdin read when piped (no spurious prompt).

Files

Area	File	Change
core	settings.ts	new exported buildLoginSettings()
core	index.ts	re-export buildLoginSettings
cli	commands/login.ts	new — runLogin, hidden-input reader, help
cli	cli-args.ts	command/login fields, parseLoginArgs, Commands: epilog
cli	cli.tsx	main() dispatches login before TUI
tests	settings-and-notify.test.ts	3 buildLoginSettings tests
tests	cli-args.test.ts	12 login parse + regression tests
tests	login.test.ts	new — runLogin write/preserve tests

Verification

• npm run typecheck — pass (core + cli + vscode)
• npm run lint — 0 new issues
• npm test — core 250 pass (+3 new), cli 264 pass (+14 new)
• Manually verified: --help, --api-key write, preserve-existing-fields merge, piped stdin, error exit on missing --api-key value, and that deepcode/-p/-r still behave as before.

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