Add sdd init orphan cleanup of installed legacy skills

[hlubek]

Greptile Summary

This PR extends sdd init to discover and remove SDD-stamped installed skill files that are no longer present in the embedded bundle.

• Adds orphan discovery and ownership/content-hash classification.
• Preserves modified or future-version files unless forced.
• Reuses the dropped-agent pruning path and reports orphan cleanup through a new callback and presenter.
• Adds integration coverage for removal, modification preservation, version downgrades, and foreign skills.
• The new recursive discovery can make initialization fail on an unreadable foreign Markdown file.

Confidence Score: 4/5

The PR should not merge until orphan discovery no longer aborts initialization because of unreadable files that SDD does not own.

The recursive scan introduces a concrete initialization failure path by reading all foreign Markdown files before ownership classification; the remaining cleanup, downgrade protection, and refactoring behavior appears sound.

Files Needing Attention: internal/finders/skill.go

Important Files Changed

Filename	Overview
internal/finders/skill.go	Adds recursive orphan discovery, but reads foreign Markdown files before excluding them and therefore propagates unrelated access failures.
internal/handlers/handler_init.go	Adds the post-install orphan sweep and consolidates safe removal behavior with existing agent pruning.
internal/model/skill.go	Adds stamp-based orphan classification and a semantic-version downgrade guard.
internal/handlers/handler_init_orphan_test.go	Covers the principal cleanup and preservation cases, but not unreadable foreign files.
internal/presenters/init.go	Adds orphan-removal output and extracts shared modified-file reporting.
internal/command/init.go	Adds the orphan-pruning callback and a helper for determining whether a result should be reported.

Prompt To Fix All With AI

### Issue 1
internal/finders/skill.go:87-90
**Foreign files abort initialization**

The orphan scan reads every Markdown file under the skills directory before determining whether SDD owns it. If a user-owned `.md` file is unreadable because of permissions or an I/O error, that error propagates through `SkillStatus` and aborts `sdd init`, even though foreign files are explicitly outside the cleanup operation.

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For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

_{Reviews (1): Last reviewed commit: "sdd: summarize 20260905-112554-s-tac-3ja..." | Re-trigger Greptile}

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