feat(skills): add shared MCP compatibility facade

[jmagar]

Summary

Adds a transport-neutral Agent Skills compatibility layer on top of Labby's existing native SEP implementation.

• extracts canonical first-party/proxied Skills list/get/read semantics into crate::skills
• exposes exactly one fixed compatibility service/tool, skills, with skills.list, skills.search, skills.get, and skills.read
• adds thin CLI and authenticated HTTP adapters over the same shared registry
• propagates caller authorization + route-visible upstream scope across the private Code Mode in-process hop and fails closed when propagation is incomplete
• preserves first-class Skills/Loadouts behavior and protected-route expose_skills gating
• hardens URI ownership/collision behavior for supporting files and unlisted skills.get
• refreshes generated service documentation and SEP provenance evidence

Security / compatibility decisions

• no per-skill MCP tools; tool cardinality is constant
• native SEP remains preferred; the fixed skills tool is the fallback for older/tool-only clients
• list/search are metadata-only; bodies are returned only by explicit skills.read
• list/search/get/read require lab:read, lab, or lab:admin; help/schema remain catalog introspection
• protected route/loadout expose_skills=false hides the fallback tool and rejects forged direct calls
• proxied reads remain manifest-bound and digest-verified
• collision-excluded skills poison all owned URIs; unlisted candidates cannot overlap published/poisoned ownership

Final landing evidence

• final PR head: a0dd33f1a12d1e2029506ff7c465fe4a96d214c6
• final base: fc4d3a1c2ad4f1e6ca1c50bded4b7d7e72b41095
• squash-merged as: ea07f3609926879da146ea6e3e3afdd65784a73a
• final GitHub checks: 39 completed, 29 success, 10 intentional skips, 0 failures, 0 pending
• final-head GitHub gates green: Clippy, Rustdoc, Format, Cargo Deny, Repository Contract, Generated docs, MCP focused regressions, MCP 2026-07-28 conformance, Code Mode runner smoke, and Feature slice (gateway)
• exact gateway feature-slice local run on the final rebased runtime: 1272/1272 passed, 3 skipped
• focused Skills tests: 52/52 passed
• Skills runtime contract conformance: 8/8 passed
• no-default-features Skills library check: green
• workspace Clippy with -D warnings: green before the final docs/CI-only base rebase; final-head GitHub Clippy also green
• legacy stdio compatibility smoke covered 2024-11-05, 2025-03-26, 2025-06-18, and 2025-11-25; all negotiated correctly, exposed the single fixed skills fallback, and completed list/get/read. The final base rebase changed docs/CI only and the final branch-only change was a test feature gate, so runtime compatibility code was unchanged afterward.

Adversarial review findings resolved

1. explicit MCP safety annotation / permanent-tool drift guard for the new read-only service
2. API help/schema auth parity with MCP
3. supporting-file/unlisted URI collision ownership gap
4. protected-loadout expose_skills=false compatibility-tool bypass
5. native Skills loadout-list import integration after rebase
6. gateway-only CI incorrectly running a Skills-only loadout regression; the test is now feature-gated and the exact gateway slice is green

Full-suite timing note

A later low-concurrency full-workspace stress pass exposed intermittent scheduler-sensitive failures in unchanged gateway pool timeout tests. The affected files have no branch diff, isolated/repeated runs passed or showed timing sensitivity, and no unrelated runtime changes were made. The required final-head GitHub test/gateway jobs are green.

Living implementation tracker: docs/plans/skills-over-mcp-compat/PROGRESS.md. The merged tracker reflects the pre-landing checkpoint; this PR body is the authoritative final landing addendum because the PR was concurrently merged before a tracker-only follow-up could be pushed.

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