[Bug]: Plugin-provided Claude skills (e.g. mattpocock-skills) never appear in the `$` picker

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

apps/server

Steps to reproduce

1. Install a plugin that ships skills (e.g. mattpocock-skills) and enable it at the project level:

   // <project>/.claude/settings.json
   { "enabledPlugins": { "mattpocock-skills@claude-plugins-official": true } }
   

2. Open the project in T3 Code (desktop AppImage), start a thread, and type $ in the composer.
3. Observe: no mattpocock-skills:* entries appear in the picker, even though Claude Code itself lists them.
4. For comparison, a skill placed under <project>/.claude/skills/<name>/SKILL.md does appear in the picker.

Expected behavior

Skills contributed by enabled plugins should appear in the $ picker alongside filesystem skills, since the engine loads them in the running session.

Actual behavior

Plugin-provided skills are never listed. The $ picker is fed from the provider snapshot's skills field, which discoverClaudeSkills (apps/server/src/provider/Drivers/ClaudeSkills.ts, ~L102) builds by scanning only two filesystem roots:

const roots = [
  { directory: path.join(configDirPath, "skills"), scope: "user" },
  ...(cwd ? [{ directory: path.join(cwd, ".claude", "skills"), scope: "project" as const }] : []),
];

Neither root covers the plugin cache, so ~/.claude/plugins/cache/claude-plugins-official/mattpocock-skills/1.2.3/skills/** is invisible to the picker.

This is picker/discovery-only: the skill is still fully invocable by name. I captured the provider init handshake for a thread with the plugin enabled and it lists mattpocock-skills:grill-with-docs (and all other plugin skills) in the model's loaded skills, with the Skill tool present. So the engine has them; only T3's filesystem inventory misses them.

Impact

Minor bug or occasional failure

Version or commit

0.0.32-nightly.20260807.1023 (desktop AppImage), Linux x64

Environment

Ubuntu (kernel 7.0.0), T3 Code desktop AppImage 0.0.32-nightly.20260807.1023, Claude provider (claude-agent-sdk 0.3.223), mattpocock-skills 1.2.3

Logs or stack traces

Provider init handshake (session claude/system/init, trimmed to the relevant fields):

{
  "skills": [ "mattpocock-skills:grill-with-docs", "mattpocock-skills:tdd", "..." ],
  "tools": [ "Skill", "..." ]
}

Workaround

Invoke plugin skills by name in prose ("use grill-with-docs on this") rather than via the $ picker — the model resolves and runs them. Picker-based discovery for plugin skills is unavailable.

Notes

Same underlying discovery limitation as #4544 (packaged desktop, project-level skills) and #5487 (.agents/skills root), but specifically for plugin-contributed skills. PR #5488 proposes adding a .agents/skills scan root; plugin skills would need a third source (the enabled plugin cache dirs) or surfacing the SDK init skill list into the snapshot instead of filesystem-only discovery.

Start in apps/server/src/provider/Drivers/ClaudeSkills.ts around discoverClaudeSkills and trace how its filesystem inventory becomes the provider snapshot's skills field. Compare that inventory with the Claude provider init handshake and enabled plugin cache paths; done means enabled plugin-provided skills appear in the $ picker alongside filesystem skills without breaking existing project skills.