feat(config): rename plugin pin to ref

[christso]

Summary

• rename workspace plugin pin to the Git-native ref
• rename skill add --pin to --ref and JSON/sync provenance pinnedRef to requestedRef
• reject obsolete plugin-object fields instead of silently following the default branch
• document symbolic ref semantics and migrate the engineering example terminology in the follow-up PR

Breaking change

Clean cutover; no compatibility alias or schema-version bump. Replace pin: with ref: and --pin with --ref. Inline owner/repo@ref sources are unchanged.

Verification

• focused ref/schema/update/path tests: 104 passed
• full Bun 1.3.12 suite: 1,457 passed, 5 skipped, 0 failed
• lint, typecheck, and build passed
• isolated workspace sync resolved compound-engineering-v3.23.3 to 871996c4e35512e3d33cdc9c2532395f7bf10831 and persisted requestedRef
• actual CLI help exposes --ref; obsolete --pin is rejected

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