feat(server): expose Grok skills in provider snapshots

[rksfn]

What Changed

Grok provider snapshots did not include filesystem skills, leaving the existing $ picker empty when Grok was selected.

• Extracted Claude's filesystem scan into discoverSkillsFromRoots, with best-effort frontmatter parsing and later-root collision precedence.
• Kept Claude's existing config and project root resolution on the shared scanner.
• Added Grok user and project roots for both .agents/skills and .grok/skills.
• Passed the server cwd and filesystem services into the Grok status probe, and retained skills on snapshots after a successful version probe even when ACP model discovery fails.
• Added focused scanner, root-order, snapshot, and Claude regression coverage.

Why

Grok can use skills from these standard filesystem locations, but T3 only exposed Claude's discovered skills to clients. Sharing the scan keeps the parsing and override behavior consistent while leaving provider-specific root resolution at the adapter boundary.

UI Changes

No client UI code changed. The existing $ picker now receives Grok skills through the provider snapshot contract.

Verification

• vp test run ... — 31 focused provider tests passed
• vp run typecheck — passed across the workspace (pre-existing Effect suggestions only)

Checklist

• This PR is small and focused
• I explained what changed and why
• Screenshots are not applicable because no client UI was changed
• Video is not applicable because no animation or interaction was changed

Implemented with GPT-5.6 Codex in the T3 Code harness.

[rksfn]

Closed because this PR was opened against the fork by mistake. The upstream PR is pingdotgg#5960.