feat: slash command palette in chat composer

[NOirBRight]

What

Adds a command palette to the pi-web chat composer that shows slash command suggestions when the user types / in the chat input.

Changes

Backend (internal/server/)

• commands.go: New /api/commands endpoint returning JSON array of {name, description} objects. Reads from ~/.pi/agent/pi-web/commands.json if available, falls back to 8 built-in defaults.
• commands_test.go: Tests for GET handler, POST rejection, file-based loading, and fallback to defaults.
• server.go: Register /api/commands route.

Frontend (web/src/session/chat/)

• command-palette.js: New module — setupCommandPalette() that:
  • Loads commands from /api/commands (5-min cache, eager load)
  • Shows a suggestion list when / is typed, filters as user continues typing
  • Arrow key navigation, Enter/Tab to select, Escape to dismiss
  • Portal-based: palette appended to document.body with position: fixed and z-index: 99999 to escape parent stacking contexts
  • Dynamically positions above the chat input via getBoundingClientRect()
• command-palette.test.js: 5 tests covering null input, fetch, filter, hide, and selection.
• chat-composer-runner.js: Import and call setupCommandPaletteForSession() during init.

Styling (internal/ui/live_templates/styles/session.css)

• Themed classes using var(--container-bg), var(--dim), var(--accent), var(--text), var(--muted)
• Mobile-aware max-height via @media (max-height: 600px)

Extension (.pi/extensions/pi-web.ts)

• On load, calls pi.getCommands() after a 3-second delay (to allow other extensions to register), then writes commands.json for the Go server to read. Refreshes every 5 minutes.

How to test

1. Start pi-web and open a session
2. Type / in the chat input — command suggestions should appear above the input
3. Continue typing to filter (e.g., /com → shows /compact)
4. Arrow keys to navigate, Enter/Tab to select, Escape to dismiss
5. Verify palette appears above all UI elements (z-index fix)
6. Verify palette follows theme colors on dark/light mode

Screenshots

Command palette showing filtered suggestions above the chat input, correctly layered above all UI elements.