WIP — not for merge yet. Implements the core of the custom Pi harness (R14/R15):
- runtime/pi/mosaic-extension.ts: native heartbeat — writes the same .hb contract
(ts/pid/status[/model]) on a MOSAIC_HEARTBEAT_INTERVAL timer; turn_start/turn_end
flip status busy/ok; model self-report via ctx.model; touches a .hb.native
precedence marker. Also FIXES a latent bug: session_end -> session_shutdown (the
old handler never fired) + corrects the import scope to @earendil-works/pi-coding-agent.
- start-agent-session.sh: sidecar DEFERS when the .hb.native marker is fresh
(< 2x interval), else writes the fallback — native precedence, sidecar fallback,
same contract so fleet ps is agnostic (per Lead's design). Generated script
validated (bash -n) + deferral/fallback behavior tested.
REMAINING before PR: surface model in `fleet ps` (parseHeartbeat + row); vitest for
the native-HB writer; "proper tool usage" (registerTool) piece; rebase onto #599's
%q sidecar (overlap on the printf line).
Refs #588
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
