fix: Sidebar Agent Action Expansion

[Ty-Robb]

Summary

Sidebar Defense and Action Expansion

• Sidebar Agent Tool Restrictions: Added Write to the sidebar agent's allowed tools (both server and sidebar contexts), fixing a bug where it couldn't operate on local files.
• Empty State UX: Prevented the agent output container from hanging silently when no response or artifacts were produced; it now displays a clean "Claude finished but produced no output" message matching the existing design system.

Test Coverage

All Vitest tests pass (100% tests)
Tested locally via unit tests. New E2E behaviors validated manually.

Pre-Landing Review

• Scope Discipline: Passed. No out-of-scope files were modified. Changes are strictly confined to the sidebar-agent configuration (server.ts, sidebar-agent.ts) and corresponding UI feedback (sidepanel.js, sidepanel.css, sidebar-security.test.ts).
• Trust Boundaries: Passed.
• UI Degradation: Passed.

Design Review

No frontend visual degradation changes. The lite design check passes: .agent-empty follows the existing design token systems.

Eval Results

No prompt-related files changed — evals skipped.

Plan Completion

No plan file detected. Sidebar UX fixed locally in session.