feat: add ralph-starter sync command for Linear status sync

[rubenmarcus]

Summary

Add a ralph-starter sync command that checks GitHub PR merge state and updates Linear issue status accordingly.

Usage

ralph-starter sync [--source linear] [--project owner/repo]

How it works

1. Query Linear for issues with status "In Progress" in the ralph-starter project
2. For each issue, check if the issue identifier (e.g. ENG-42) appears in any open/merged GitHub PR via gh pr list --search "ENG-42"
3. If PR is merged → update Linear to "Done" + add comment with PR URL
4. If PR is closed (not merged) → add comment "PR closed without merge"
5. If PR is open → keep "In Progress"

Implementation

Files to create/modify

• src/commands/sync.ts — New sync command (~150 lines)
• src/cli.ts — Register sync command
• docs/docs/cli/sync.md — Documentation

Dependencies

• Uses WritableIntegration from PR feat: unified task management across GitHub & Linear #196 for Linear mutations
• Uses gh CLI for GitHub PR queries
• Reads LINEAR_API_KEY from env or ralph-starter config

Acceptance criteria

• ralph-starter sync queries Linear and syncs status
• Merged PRs → Linear "Done" + comment
• Closed PRs → Linear comment
• Open PRs → no change
• Dry-run mode: ralph-starter sync --dry-run
• Tests for sync logic