fix: add Write tool to sidebar agent + surface empty responses

[sternryan]

Summary

• Adds Write to --allowedTools in both server.ts (line 433) and sidebar-agent.ts (line 165), enabling the sidebar agent to create files (CSVs, exports, etc.) when users ask
• Surfaces a "Done (no text output)" notice in the sidebar UI when Claude finishes without producing text output, instead of leaving an empty response bubble
• Adds .agent-empty CSS class for the notice styling

Addresses the P1 TODO: "Sidebar agent needs Write tool + better error visibility"

Changes

File	Change
browse/src/server.ts	Add Write to --allowedTools
browse/src/sidebar-agent.ts	Add Write to fallback --allowedTools
extension/sidepanel.js	Show "Done (no text output)" when agent_done fires with no agentTextEl
extension/sidepanel.css	Add .agent-empty style (muted, italic, 12px)

Test plan

• bun test test/skill-validation.test.ts test/gen-skill-docs.test.ts -- 601/601 pass
• Manual: ask sidebar to "write a CSV file" and verify it creates the file
• Manual: send a message that produces only tool output (no text) and verify the "Done (no text output)" notice appears