feat: add social/announcement post generation to release-notes skill

[PureWeen]

Summary

Adds a new Step 5: Generate Social / Announcement Post to the release-notes skill.

When an agent runs the release-notes workflow, it now also generates copy-paste-ready announcement text for:

• LinkedIn (long-form) — hook + highlights + one-liner + install link + hashtags, kept under 1300 chars
• Twitter/X / Bluesky (short-form) — compact bullet format with link

Writing rules baked into the skill

• Lead with the hook, not the version number
• Benefits, not implementation details
• Include a one-liner for readers who don't know PolyPilot
• Hashtags at the end, never inline

Example output (v1.0.15)

🚀 PolyPilot v1.0.15 is out!

You can now spread your AI coding agents across every machine on your LAN.

What's new:
🔹 Fiesta Mode — discover other PolyPilot instances, pair in one click, and dispatch tasks to any machine
🔹 Mixed-Model PR Review — 5 workers each run 3 sub-agents and synthesize consensus reviews
🔹 CLI Agent Visibility — see when Copilot invokes specialized agents, plus a new /agent command

Follows up on #458 (merged). No code changes — skill doc only.

[PureWeen]

Code Review — PR #461

Build: ⚠️ No CI checks reported (docs-only branch) | Risk: None (docs-only)

Multi-Model Review (Opus + Sonnet + Codex)

Consensus: All 3 models agree this PR is clean. No regressions, security issues, bugs, data loss risks, race conditions, or code quality problems.

Verified by all models

• Step renumbering is correct and complete (new Step 5 inserted; old 5→6, 6→7)
• No broken cross-references — no other files reference step numbers
• No executable code changed — purely documentation
• No security concerns — URLs point to public GitHub releases

Non-consensus observations (1 model only — informational, not blocking)

Sonnet flagged three minor items that did not reach the 2/3 consensus threshold:

1. Skill frontmatter description — doesn't mention social post generation, so skill discovery may not trigger for "write a LinkedIn post" queries
2. LinkedIn 1300-char limit — the actual LinkedIn feed preview truncates at ~210 chars; the 3000-char hard limit is the real platform boundary
3. Short-form template missing char limits — Twitter/X is 280 chars, Bluesky is 300; the template could exceed these without guidance

These are valid quality observations but are not bugs or regressions.

Prior review comments

One prior comment from PureWeen on the parent PR (#458) — suggestions about test count, TestFlight, dropped features, and Linux asset naming. Those apply to #458, not this follow-up.

✅ Verdict: Approve

Documentation-only PR adding a well-structured social post generation step. No code changes, no risk. Ship it.