fix(pi): detect managed extension installs

[guillemservera]

Summary

• detect context-mode installed by Pi under <agent-dir>/npm/node_modules
• honor PI_CODING_AGENT_DIR for custom Pi agent directories
• retain ~/.pi/extensions/context-mode as a legacy fallback
• avoid advertising the legacy directory as the only hook location

This fixes context-mode doctor reporting a missing Pi extension after pi install npm:context-mode.

Validation

• npx vitest run tests/adapters/pi-adapter.test.ts (20 tests passed)
• npm run typecheck
• git diff --check