feat: add Pi session provider

[0dragosh]

Summary

• add Pi as a core session provider alongside Claude and Codex
• make session discovery and transcript parsing provider-aware for Pi session files
• wire Pi through launch/resume, remote checks, app provider switching, docs, and test coverage

Verification

• cargo check
• cargo test

CI

• manually dispatched CI for feat/pi because the workflow only auto-runs on main pushes and PRs to main
• current run 24788164435: Tests passed, Format failed, and Check & Clippy failed