Add daemon-backed Pi npm plugin

[Stephen Belanger (Qard)]

Summary

• import @braintrust/pi-extension as monorepo-owned package content while preserving its version, docs, fixtures, and compatibility tooling
• replace the shipped Pi tracing state machine with the shared fail-open daemon client while retaining tracing status and permalink UI
• send only a non-secret session route: optional profile and organization selection plus a typed destination; omitted profile is resolved as the default by the daemon host
• keep project selection inside the destination instead of separate project, parent-span, or root-span fields
• add a Pi Rust translator for turns, LLMs, streaming timing, tools, skills, compaction, branch summaries, attachment, shutdown, and recovery
• remove the Braintrust SDK and local span state from the runtime tarball; the imported processor remains test-only for fixture parity
• add pnpm tarball build/validation/publish safeguards and a rolling five-minor Pi compatibility matrix
• run the actual Pi CLI through the shared integration harness with deterministic model inference, the built extension tarball, the daemon, and mock Braintrust ingest

Validation

• make validate-pi (6 files, 60 tests, package build, smoke import, and tarball validation)
• cargo test --manifest-path bt-daemon/Cargo.toml --all-features --locked
• cargo clippy --manifest-path bt-daemon/Cargo.toml --all-targets --all-features --locked -- -D warnings
• PI_BIN=... PI_EXTENSION_PATH=... BT_AGENT_INFERENCE_MODE=mock BT_AGENT_INGEST_MODE=mock cargo test --manifest-path bt-daemon/Cargo.toml --all-features --locked --test agent_integration pi_session_emits_traces -- --ignored --nocapture --test-threads=1
• clean tarball install with @earendil-works/pi-coding-agent@0.80.10; pi --version reported 0.80.10
• compatibility resolver selected 0.79.10, 0.80.10, 0.81.1, 0.82.1, and 0.83.0

Release safety

No npm package, marketplace repository, tag, or GitHub release was published. Real npm publishing requires an explicit release confirmation.

Stack

This PR intentionally targets the OpenCode integration branch and should merge only after the OpenCode foundation.