feat: add Pi supervised question bridge

[stoneymarrow]

Summary

• add the versioned Firstmate-owned Pi question request, response, owner, resolution, and locking protocol
• wire supervised Pi crew launches to the bridge and add guarded answer, startup recovery, timeout, cancellation, reissue, and teardown handling
• document the schema and operator workflow, including keyed status folding for the report-defined suffix format
• keep Phase 6 distribution, canary, and primary experiments out of scope

Validation

• bash tests/fm-pi-supervised-question.test.sh
• bash tests/fm-pi-watch-extension.test.sh
• bash tests/fm-spawn-dispatch-profile.test.sh
• bash tests/fm-bootstrap.test.sh
• bash tests/fm-session-start.test.sh
• bash tests/fm-teardown.test.sh
• bash tests/fm-watch-triage.test.sh
• bash tests/fm-decision-hold-lifecycle.test.sh
• bin/fm-lint.sh
• git diff --check

[stoneymarrow]

CI follow-up: pushed commit 89315d3 to fix the Linux behavior-test failure. GNU stat accepts -f as filesystem reporting, so the old BSD-or-GNU fallback never reached stat -c; the test now selects mode syntax by platform. The focused Pi bridge suite, simulated Linux mode path, ShellCheck, and git diff --check pass locally. The remaining no-mistakes check requires a PR-body signature from that pipeline; this repair task was explicitly instructed not to run or claim that pipeline.