feat(trace): support Pi coding agent sessions

[waveywaves]

Design

• spec: 005 Pi coding agent support in Chainloop Trace #3523 — Spec 005: Pi coding agent support in Chainloop Trace

Problem / use case

Chainloop Trace can capture coding sessions from Claude Code, Cursor, and OpenCode, but not from Pi. Work produced with Pi therefore cannot provide native Chainloop AI coding-session evidence and currently needs the skip-ai-session bypass.

Pi exposes project-local TypeScript extensions, session lifecycle events, tool-call events, and persisted JSONL v3 session files. Those capabilities are sufficient for a native Trace provider without changing Pi itself.

Expected outcome

• chainloop trace init --pi and chainloop trace run --pi install Pi support alongside the existing providers.
• Chainloop installs one marked, project-local Pi extension, refuses unrelated content at the same path, and removes only its own file during uninstall.
• Pi session start, shutdown, write, edit, and bash events feed the existing Trace hook pipeline without blocking agent work.
• The provider copies Pi's persisted session JSONL before parsing it and emits the existing AI coding-session evidence type.
• Pi session trees, model/provider metadata, token usage, tool counts, and conversation counts are represented without changing the control-plane API or attestation schema.
• Pi receives the full session instruction once, the Spec 003 reminder on every user turn, and post-push links through UI or headless stderr.
• Pi reuses the existing tool_use_id; file snapshots adopt the same per-call correlation already used by command snapshots. Existing provider defaults and external behavior remain unchanged.

Constraints

• Follow the repository's spec-first process before implementation.
• Do not require a Pi fork or a separately published Pi package.
• Keep the generated extension functional in interactive, RPC, JSON, and print modes without writing arbitrary output to Pi's stdout.
• Treat missing transcripts, abrupt process termination, and legacy or unknown JSONL fields as best-effort conditions that must not block Pi or a Git push. Hook children use a 60-second timeout, then SIGTERM with five seconds of grace before SIGKILL.