feat: add Pi ready marker and session metadata

[RogerNavelsaker]

Summary

• pass explicit Overstory session metadata into Pi-managed sessions
• switch Pi readiness detection to the explicit ✓ os-eco agent=... runtime=pi marker emitted by the companion extension
• remove legacy core-generated Pi guard files and rely on the external os-eco Pi extension for session-scoped policy

Testing

• bun test src/runtimes/pi.test.ts src/commands/sling.test.ts src/commands/coordinator.test.ts

[jayminwest]

Review

Good DRY refactor centralizing session env vars, and the Pi ready marker is a clear improvement over fragile TUI heuristic detection. A few issues to address:

Must fix

1. Supervisor sessionKind is wrong (src/commands/supervisor.ts) — The supervisor gets sessionKind: "coordinator" but it's a per-project supervisor, not a coordinator. The OverstorySessionKind union includes "standalone" | "orchestrator" | "coordinator" | "monitor" | "worker" but omits "supervisor". Either add "supervisor" to the union or use a more semantically accurate kind. This propagates an incorrect OVERSTORY_SESSION_KIND=coordinator env var to any consumer.

2. No tests for buildOverstorySessionEnv() (src/agents/session-env.ts) — This is a new pure function with conditional spread logic. It should have a colocated session-env.test.ts covering: (a) all required fields are set, (b) optional taskId/profile are omitted when undefined and included when defined, (c) baseEnv merge order (Overstory keys should override baseEnv on collision). Trivially testable pure function — should be straightforward.

3. Types belong in src/types.ts per project convention — CLAUDE.md states "All shared types and interfaces go in src/types.ts". The OverstorySessionKind type and OverstorySessionEnvOpts interface are cross-cutting (used by coordinator, monitor, supervisor, sling) and should be moved there.

Non-blocking suggestions

• Document that OVERSTORY_SESSION_KIND, OVERSTORY_CAPABILITY, and OVERSTORY_PROJECT_ROOT are part of the external contract with the os-eco Pi extension (no consumers in the overstory codebase yet).
• The PI_READY_MARKER_PREFIX uses a Unicode checkmark (\u2713) — consider a comment noting the exact character for searchability.
• sapling.ts:48 comment "Mirrors the constant in pi-guards.ts" — the cross-reference was removed but no replacement context was added.

[RogerNavelsaker]

Rebased onto current main and force-pushed. This branch now supersedes #112 for the Pi startup/readiness line of work: it carries the newer explicit ready-marker + session-env contract, the supervisor session-kind fix, direct tests for buildOverstorySessionEnv(), and the follow-up docs/comments requested in review.

[RogerNavelsaker]

Hi @jayminwest,

as I’ve been working on this, I realized it might be cleaner to keep the os-eco related tools together. I originally created the pi-os-eco extension as a way to contribute while keeping the Overstory code separate from the direct pi-coding-agent.

​If you're interested, I’m happy to transfer the pi-os-eco repository to you. This would allow you to publish the extension to NPM under your own organization and keep the entire ecosystem under one roof. Let me know if that’s something you’d like to do.