feat: project-orchestrator — engine@v0.1.0 submodule, PO config, fleet.toml registry, mgmt scripts, docs, Nix

The PO is itself a project using the agent-orchestrator harness (engine/ submodule pinned at
v0.1.0). Adds: agents.toml (one persistent fleet-management agent) + prompts/; fleet.toml (the
sole project<->harness<->ref registry) + docs/fleet-registry.md; scripts/ (fleet.py +
create/start/stop/update-project.sh); docs/manage-projects.md + docs/bootstrap.md; flake.nix/.lock
devShell (python311+tmux+git); README.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

prompts/supervise.md

@@ -0,0 +1,17 @@
+# Periodic fleet sweep
+
+A scheduled wake. Do a light, read-only sweep of the fleet — do not start work unless something is
+clearly wrong and a runbook covers the fix.
+
+1. `python3 scripts/fleet.py status` — list every project in `fleet.toml` with its location, harness,
+   pinned ref, and enabled flag.
+2. For each **enabled** project whose location is reachable from this host, optionally check whether
+   its harness reports it running (for an `agent-orchestrator` project:
+   `engine/agents.py status --config <project>/agents.toml`). Reading its harness docs first if the
+   harness is unfamiliar.
+3. Report a one-paragraph summary: total / enabled / disabled, anything unreachable or stopped that
+   should be running. If a fix is needed and `docs/manage-projects.md` covers it, you may apply it;
+   otherwise just flag it.
+
+Remember the one-directional rule: never write fleet/PO state into a project repo. The fleet's truth
+is `fleet.toml` here.