Copilot Chat cannot resolve installed custom skills

Does this issue occur when all extensions are disabled?: Yes/No

- VS Code Version: 1.136.0
- OS Version: macOS 26.6.3
- Feature: Agent mode
- Selected model: Claude Sonnet 5
- Logs: Attached, with credentials and personal paths redacted

## Steps to Reproduce

1. Install or make the custom skill `executing-plans` available in the configured skills directory.
2. Open the workspace in VS Code.
3. Start Copilot Chat in Agent mode.
4. Ask Copilot to use or invoke the `executing-plans` skill.
5. Register more than 15 skills, start a new VS Code Agent session, inspect the reported/usable skills, then ask Copilot to invoke one outside the active subset.
6. Observe the tool invocation error.

- The issue affects workspaces/sessions with more than 15 available skills; testing showed that approximately the first 15 skills were actively registered and usable, while the exact threshold varied slightly by model.
- The behavior was observed with VS Code Agent Mode on Windows across multiple models.
- Skills beyond the active subset can still appear present or be recognized by name, but automatic invocation fails. The related error is visible in Output -> GitHub Copilot Chat.

## Actual behavior

Copilot invokes:

```text
Error from tool skill with args {"skill": "executing-plans"}:
Skill "executing-plans" not found.
Available skills: agent-creator, airunway-aks-setup, appinsights-instrumentation, azure-ai, ...
at $Da (/Applications/Visual Studio Code.app/Contents/Resources/app/extensions/copilot/dist/extension.js:5807:1584)
at hLe.resolveSkillUri (/Applications/Visual Studio Code.app/Contents/Resources/app/extensions/copilot/dist/extension.js:5807:1090)
at hLe.invoke (/Applications/Visual Studio Code.app/Contents/Resources/app/extensions/copilot/dist/extension.js:5791:13642)
at jP.$invokeTool (file:///Applications/Visual%20Studio%20Code.app/Contents/Resources/app/out/vs/workbench/api/node/extensionHostProcess.js:654:4184)
```

The skill exists and is installed, but it is not included in the resolver's available-skill list.

The same behavior occurs with another locally available custom skill. This suggests that the skill resolver or index may be loading only a partial catalog instead of discovering all configured workspace or user skills.

## Expected behavior

Copilot should discover and invoke the installed skill.

If the skill cannot be loaded, the error should identify:

- The directories that were searched
- Whether the skill metadata was invalid
- Whether the skill location is unsupported
- Whether the skill index is stale

## Additional information

- The failure occurs during the `skill` tool invocation.
- Restarting VS Code and reloading the workspace did not resolve the issue.
- The available-skill list appears limited to Azure-related skills.
- The skill metadata follows the documented `SKILL.md` format.