[feat] Commit-to-Linear comment write-back

[Sph3ricalPeter]

Priority: P2 — opt-in, potentially noisy
Effort: ~2–3 days
AI required: No
Depends on: Linear API integration foundation

When a new commit lands with a Linear issue ID, optionally post a comment to that issue: Commit <sha> on branch <branch>: <commit subject>. Gives traceability without a PR.

Scope:

• Off by default. Opt-in per repo and per issue-ID pattern (e.g., only for issues in team ENG)
• Triggered on commit (local) and/or push (remote) — user choice
• Comment format configurable via template
• Batch mode: if pushing 10 commits, post one consolidated comment instead of 10 separate ones
• Preview-before-send option for cautious users
• Throttle to avoid hammering Linear API

Acceptance criteria:

• Off by default with clear setup flow
• Trigger choice (on commit / on push) configurable
• Batch mode for multi-commit pushes
• Throttling prevents API abuse
• Easy global disable for "this feels too noisy"