feat: expose skills as slash commands with skill fallback execution

[hannesrudolph]

Summary

• Added autogenerated discovery of skill-backed slash commands so available skills are exposed in slash command flows.
• Implemented invocation fallback behavior in mention/tool execution paths so skill commands can resolve and execute when direct slash command handlers are absent.
• Enforced command precedence so real slash commands continue to win before skill fallback execution is attempted.

Validation

• cd src && pnpm -s check-types ✅ PASS
• cd src && npx vitest run __tests__/command-mentions.spec.ts core/mentions/__tests__/index.spec.ts core/mentions/__tests__/processUserContentMentions.spec.ts core/tools/__tests__/runSlashCommandTool.spec.ts core/webview/__tests__/webviewMessageHandler.spec.ts ✅ PASS

Interactively review PR in Roo Code Cloud

[hannesrudolph]

@roomote run the pr review on this

[roomote-v0]

Fixaroo   See task

Reviewed the PR and addressed two issues: added missing unit tests for skillInvocation.ts, fixed inconsistent optional chaining on getSkillsManager in RunSlashCommandTool.ts, and added getSkillsManager to the default test mock so command-not-found tests work without relying on ?. method calls. All checks pass (type-check, lint, 89 tests).

_{View commit | Revert commit}

[hannesrudolph]

Completed review of this PR. No actionable issues were identified within the changed scope.