feat: add Pi agent support

[gbasin]

Summary

Adds support for the Pi coding agent alongside existing Claude and Codex support.

Changes

Backend

• Log discovery (logDiscovery.ts): Scan ~/.pi/agent/sessions/ for Pi JSONL logs, with PI_HOME env override support
• Session detection (logMatcher.ts): Extract user messages from Pi TUI using ANSI background color detection (RGB 52,53,65 from tokyo-night theme)
• Agent detection (agentDetection.ts): Recognize pi command
• Database migration (db.ts): Add pi to agent_type CHECK constraint with safe table recreation

Frontend

• Pi icon (AgentIcon.tsx): New π symbol icon for Pi sessions
• Settings preset (settingsStore.ts): Add built-in "Pi" command preset with migration for existing users
• Types (types.ts): Add pi to AgentType union

Tests

• Updated all test fixtures to include PI_HOME environment variable
• Added tests for Pi TUI user message extraction
• All 435 tests pass

Notes

• Pi user message detection relies on the tokyo-night theme's specific background color (RGB 52,53,65). If Pi changes this color or users select a different theme, detection will gracefully fail (empty array returned).
• Session ID extraction uses entry.type === "session" with entry.id field per Pi's JSONL format.

Checklist

• Lint passes
• Typecheck passes
• All tests pass
• Database migration is safe (transactional table recreation)
• Settings migration handles existing users