ca-pi: child dispatch has no gate-events audit trail; supervisor failures reasonless (observability-001/002)

[SUaDtL]

Branch: feat/pi-support · Tribunal run 2026-07-17-pi-support-branch · Finding: findings/observability/*.json (group pi-child-audit)
runner.ts/dispatch.ts write no gate-events.log line and discard child stderr, unlike bridge.ts:213-243 and compaction.ts:436-451. windows-supervisor.ts/process-tree.ts failure modes collapse to bare REFUSED/false. Fix: audit parity (task/role/model/exit/duration, bounded redacted stderr) + reason codes.

[SUaDtL]

Verified against PR #313 head 9ed248f4: dispatch.ts appends one bounded, redacted gate-events.log record for each child outcome with role, model, exit class/code, duration, stderr byte count, and a newline-folded diagnostic head. process-tree.ts, windows-supervisor.ts, and runner.ts carry stable allowlisted refusal reason codes through the failure path. The hosted Pi adapter matrix is green on Linux, macOS, and Windows for both supported runtimes. This finding is implemented on #313, which now carries Closes #315.