Split social posts per author × platform with per-platform rules

[hiroshinishio]

Summary

• Split the single ## Social Media Post section into five — GitAuto on X, GitAuto on LinkedIn, Wes on X, Wes on LinkedIn, and HN Title — so each platform gets text written for its actual reader.
• Added per-platform voice/length rules to CLAUDE.md (GitAuto X ≤280, GitAuto LI 400-800, Wes X long-form via Premium, Wes LI 600-1200, HN title ≤80).
• Fixed a LinkedIn API bug where content.article.title was set to the same string as the post body, making the preview card echo the post text back at itself. Posts are plain text now; URLs go inline when needed.
• Dropped the sns label gate on the Hacker News job. HN now fires whenever the HN Title section exists, matching the opt-in model of the other platforms.
• Renamed .github/scripts/post-twitter.js → post-x.js.
• Each section is independent: if a section is missing, only that single post is skipped — no all-or-nothing coupling.