feat(cli): add configurable plansDirectory for Plan Mode

[shenyankm]

Summary

• What changed: Added a plansDirectory setting for Plan Mode so approved plan files can be stored in a project-specific directory instead of always using the global default.
• Why it changed: This resolves Add configurable plansDirectory setting for Plan Mode (like Gemini CLI / Claude Code) #3548 by allowing users to keep plans inside the project and grant narrower write access for Plan Mode workflows.
• Reviewer focus: Please review path resolution, rejection of directories outside the project root, and the default fallback behavior when plansDirectory is unset.

Validation

• Commands run:

  cd packages/core && npx vitest run src/config/storage.test.ts src/config/config.test.ts
  cd packages/cli && npx vitest run src/config/config.test.ts src/config/settingsSchema.test.ts
  npm run build
  npm run typecheck

• Prompts / inputs used: N/A. This change was validated through focused configuration and storage tests rather than manual TUI interaction.

• Expected result:

  • Relative plansDirectory values resolve from the project root.
  • Paths outside the project root are rejected.
  • The default plan directory remains ~/.qwen/plans when unset.
  • CLI config, generated schema, docs, and plan persistence remain valid.

• Observed result:

  • Core config/storage tests passed: 197 tests.
  • CLI config/schema tests passed: 220 tests, 2 skipped.
  • npm run build completed successfully.
  • npm run typecheck completed successfully.

• Quickest reviewer verification path:

  1. Add "plansDirectory": "./.qwen/plans" to project .qwen/settings.json.
  2. Start Qwen Code from that project.
  3. Enter Plan Mode and approve a plan.
  4. Confirm the plan file is written under .qwen/plans.
  5. Change the setting to "../plans" and confirm startup fails with a configuration error.

• Evidence:

  • Automated tests cover default behavior, relative and absolute custom directories, path traversal rejection, symlink escape prevention, Windows cross-drive handling, mixed separators, CLI config wiring, schema exposure, and atomic plan writes.

Scope / Risk

• Main risk or tradeoff: Path validation is intentionally strict; configured plan directories must resolve inside the project root.
• Not covered / not validated: Manual TUI verification was not run.
• Breaking changes / migration notes: No breaking change. Existing behavior is preserved when plansDirectory is unset.

Testing Matrix

	🍏	🪟	🐧
npm run	⚠️	✅	⚠️
npx	⚠️	✅	⚠️
Docker	⚠️	⚠️	⚠️
Podman	⚠️	N/A	⚠️
Seatbelt	⚠️	N/A	N/A

Testing matrix notes:

• Validated on Windows only.
• Docker, Podman, and Seatbelt were not exercised because this change is limited to configuration resolution and plan file path handling, not sandbox behavior.

Linked Issues / Bugs

Closes #3548