Support a description file in start-planning.sh

[taspinar]

What changed

• start-planning.sh accepts --description <file>. The file may be outside the repository or uncommitted inside it; it is copied to docs/PROJECT_DESCRIPTION.md in the planning worktree before the scope baselines are taken, so neither planning phase may modify it.
• The Project Grill contract reads the description first and asks only about what it leaves unresolved; the planner contract treats it as background below the approved requirements.
• A missing, empty, or non-regular description fails before a branch or worktree is created. An uncommitted description inside the repository is the only change the clean-checkout check allows.
• Unknown options are now rejected.
• The description is committed with the planning branch; the closing guidance lists it.
• README and docs/development.md document the option.

Issue / acceptance criteria

Closes #24

Risk

• Low
• Medium
• High

Verification evidence

• ./scripts/verify.sh passed (9 checks)
• Tests added/updated where appropriate (also passing under macOS system bash 3.2)
• Independent review completed when required
• Architecture/docs/ADR updated when required

Independent review: Codex (gpt-6-astra), read-only via review-feature.sh. Verdict CHANGES REQUIRED.

• M1 (uncommitted descriptions inside the repository were rejected by the clean-checkout check): fixed, with tests for an in-repository description and for unrelated dirty files.
• M2 (positional <agent> <model> invocation unsupported): not changed. That criterion predates Centralize agent configuration and launcher #28, which replaced the positional arguments; the superseded wording is recorded in a comment on the Issue.

Not re-reviewed after the fix.

Agent involvement

Planner: —
Implementer: Claude (Opus 5.5)
Reviewer: Codex (gpt-6-astra)

Production impact

None.

🤖 Generated with Claude Code