docs(skills): add extension deprecation to publish skill

[stack72]

Summary

• Adds extension deprecation/undeprecation documentation to the swamp-extension-publish skill
• Updates frontmatter triggers to include deprecation-related phrases
• Adds deprecation errors and related skills entry to references/publishing.md

The deprecation feature shipped in #1455 but wasn't covered in any skill. It fits naturally in swamp-extension-publish since deprecation is a registry operation sharing the same auth/collective context as push.

Passed npx tessl skill review at 97%.

Test Plan

• deno fmt passes on modified files
• npx tessl skill review .claude/skills/swamp-extension-publish passes (97%)
• Verify skill triggers on "deprecate extension" in a fresh conversation

🤖 Generated with Claude Code

[github-actions]

Code Review

Documentation-only PR adding deprecation/undeprecation coverage to the swamp-extension-publish skill. Verified against the actual CLI implementation from #1455.

Blocking Issues

None.

Suggestions

None — this is a clean, well-structured docs addition. Specific things I verified:

• SKILL.md line count: ~248 lines post-merge, well under the 500-line limit
• Frontmatter: Correct structure (name + description only), triggers updated with deprecation phrases
• CLI flag accuracy: --reason (required), --superseded-by (optional), -y/--yes all match extension_deprecate.ts and extension_undeprecate.ts
• Section placement: "Post-Publication: Deprecation" after State 8 (pushed) is logical — deprecation is a post-publish lifecycle operation
• references/publishing.md: New error entries ("Extension is already deprecated", "Extension is not deprecated") and Related Skills self-reference are accurate
• DDD: No domain code changes; the existing implementation correctly uses libswamp boundary imports