feat(source): linked (editable) sources — SPEC-007 (closes #69)

[nmccready]

Implements SPEC-007 (spec) — the npm link / go mod replace analogue for .agents/ artifacts. A linked source symlinks a managed artifact at a live local git checkout instead of a SHA-pinned tarball snapshot, so edits flow both ways and git pull in the checkout reaps updates with no re-fetch. Replaces the undocumented, non-portable manual ln -s.

What's new

sync-agents source add --link=../foo-skill skill:me/foo-skill  # link a checkout you own
sync-agents source add --link=../foo-skill                     # derive entry from git remote
sync-agents source add --link skill:me/foo-skill               # managed clone (.agents/.sources/), scanned once

Acceptance criteria (issue #69)

• Override.Link + LockEntry.Link/ManagedClone schema; link⊕pin_sha validated at load
• file: parse (relative ok, absolute rejected)
• add --link=<path> <entry> writes relative manifest/lock + relative symlink; re-pull no-op, no clobber
• add --link <entry> managed clone + SPEC-005 scan gate
• --link=<path> derives owner/repo from git remote
• update: managed clone git pull --ff-only; user checkout no-op
• index enumerates linked skill in AGENTS.md (follows symlinks)
• detach link → vendored copy (freeze; keeps upstream identity)
• portability test: no absolute paths in lock/symlink
• docs synced (README + SPEC status → Implemented)

Design notes

• All persisted paths use the file: scheme relative to .agents/; the on-disk symlink is created relative too — absolute file: paths are rejected at parse time (git-tracked sources.lock must stay portable).
• Reuses the existing Override mechanism (adds link, mutually exclusive with pin_sha).
• A linked source is a working tree the operator owns → trusted by default; only a managed clone's first fetch is scanned (matches the ClawHub supply-chain posture).
• pull never re-fetches a snapshot over a link; a moved/deleted checkout surfaces as a warning, not a silent revert.
• scan/copyTree now skip nested .git (VCS metadata is never artifact content).

Tests

• Go: file: parse table, link⊕pin_sha + absolute rejection, all three add --link forms (managed clone uses a hermetic local repo via injectable CloneURL), dangling-target warn, detach freeze, index-follows-symlink.
• bats: 7 end-to-end cases. Full suite 139/139 green; go test ./..., go vet, gofmt clean.

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Severity Levels:

• 🔴 Critical: Must fix (bugs, security, data loss)
• 🟡 Warning: Should fix (performance, maintainability)
• 🔵 Suggestion: Nice to have (style, alternatives)
• ⚪ Nit: Optional (cosmetic)