Add editable plan files feature

[magiak]

Summary

• Edit Plan button on plan messages in chat — opens the latest plan file (~/.claude/plans/*.md) in VS Code editor
• Show Plan toolbar button in the header for quick access to the latest plan file at any time
• File watcher on ~/.claude/plans/ that detects saves and prompts user to notify Claude of changes
• Deploy script (npm run deploy) for quick rebuild and install during development
• README documentation for the new feature and updated development setup

Changes

• src/extension.ts — Plan file utilities, showPlan command, file watcher setup, message handler
• src/script.ts — openPlanFile() function, isPlanContent() heuristic, Edit Plan button injection
• src/ui.ts — Show Plan button in header toolbar
• src/ui-styles.ts — Styling for plan action buttons
• package.json — New command registration, deploy script
• README.md — Feature documentation and updated dev setup

Test plan

• Open a chat session and trigger plan mode — verify "Edit Plan" button appears on plan messages
• Click "Edit Plan" — verify the latest .md file from ~/.claude/plans/ opens in VS Code
• Click "Show Plan" in the header toolbar — verify it opens the latest plan file
• Edit and save a plan file — verify notification appears with "Send Update to Claude" option
• Run npm run deploy — verify it builds, packages, and installs the extension
• Test with no plan files present — verify graceful "No plan files found" message