feat: add read-only supervision model

[JTInventory]

Summary

Firstmate now has a passive supervision command that turns current fleet signals into one stable checklist and JSON contract. This gives Firstmate and future display tools a shared firstmate.supervision.v1 model without making Radar own task-state rules or changing runtime state.

The model reads local state, status logs, tmux liveness, git worktrees, watcher proof files, treehouse status, and optional read-only GitHub PR data. GitHub and watcher proof failures degrade to unknown data, so the command keeps local visibility without pretending a merge or watcher state is proved.

Notes

• No built-in owner-upstream PR reminder is reintroduced; external PRs are opt-in with --external-pr or environment-provided defaults.
• Radar migration is intentionally left for a follow-up.
• Current route metadata (route_profile, route_harness, route_model, route_effort, and recorded branch) is preserved in task JSON.

Validation

• bash -n bin/fm-supervise.sh bin/fm-supervision-model.sh tests/fm-supervision-model.test.sh
• bash tests/fm-supervision-model.test.sh
• bin/fm-supervise.sh --json --no-default-reminders | python3 -m json.tool
• bin/fm-supervise.sh --schema | python3 -m json.tool
• bin/fm-supervise.sh --no-default-reminders
• bin/fm-supervise.sh --json --no-default-reminders --external-pr https://github.com/JTInventory/firstmate/pull/24 | python3 -m json.tool
• git diff --check

Full tests/*.test.sh was attempted. Two existing, untouched tests failed locally: tests/fm-secondmate-safety.test.sh (fm-pr-check failed under FM_HOME) and tests/fm-teardown.test.sh (could not determine head branch for PR https://github.com/example/repo/pull/7). shellcheck is not installed in this environment.

---