[feat] Acceptance-criteria checklist sync

[Sph3ricalPeter]

Priority: P2
Effort: ~3–4 days
AI required: No
Depends on: Issue context panel

Acceptance criteria parsed from Linear issue descriptions become interactive checkboxes in the issue context panel. Ticking them off triggers a comment write-back to Linear.

Scope:

• Parse markdown task lists (- [ ] / - [x]) from issue description
• Render as interactive checkboxes in the panel
• On commit, if any criteria were ticked since last commit, offer to post a comment to the Linear issue: Commit <sha>: marked criterion "<text>" as complete
• Preview-before-send for every comment — never auto-post silently
• Per-issue and per-repo opt-out

Acceptance criteria:

• Markdown task lists parsed correctly
• Checkbox state syncs to Linear API
• Comment preview shown before posting
• User can disable write-back per repo or globally
• Race conditions handled (criterion edited in Linear while user has it open)