feat: vendor add end-to-end with deterministic lockfile v2

- go-git in-process clone, default-branch/tag resolution, optional token auth
- web-tree URL normalization (github/gitlab) with implied discovery root
- discovery of skills/commands/agents/mcp, lenient frontmatter with warnings
- selection: all by default, --include with missing-entry hard errors
- external/<source>/<type>/ materialization preserving exec bits
- lockfile v2: deterministic JSON, pinned url/ref/rev, inventory, warnings
- offline black-box e2e suite against local fixture git repositories

.beads/interactions.jsonl

@@ -11,3 +11,4 @@
 {"id":"int-f444e5d1","kind":"field_change","created_at":"2026-05-28T08:13:14.820734703Z","actor":"m3ta-chiron","issue_id":"agent-lib-4wx.11","extra":{"field":"status","new_value":"closed","old_value":"in_progress"}}
 {"id":"int-b1779011c451e8aecfac8a15e2c332b8","kind":"field_change","created_at":"2026-06-13T07:55:57.404556723Z","actor":"m3ta-chiron","issue_id":"agent-lib-j6b","extra":{"field":"status","new_value":"closed","old_value":"in_progress","reason":"Architecture review report written and opened: /tmp/architecture-review-20260613-075521.html"}}
 {"id":"int-eaab83169be77c1e6a853e502b70362f","kind":"field_change","created_at":"2026-06-13T08:17:27.20786458Z","actor":"m3ta-chiron","issue_id":"agent-lib-feu","extra":{"field":"status","new_value":"closed","old_value":"in_progress","reason":"Babysitter Deployment Plan exploration completed. Artifact: .a5c/deployment-plan-exploration.md. Run: 01KV00D3F73F6KY5Q8W4TFER5X"}}
+{"id":"int-4d5bf5dc890a6963a0e1110a8fb71635","kind":"field_change","created_at":"2026-08-22T19:43:28.592284059Z","actor":"m3ta-chiron","issue_id":"agent-lib-j2w.1","extra":{"field":"status","new_value":"closed","old_value":"open"}}