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>

.gitignore

@@ -0,0 +1,9 @@
+# runtime state + logs (never committed)
+.ao-state/
+*.log
+__pycache__/
+*.pyc
+result
+
+# projects scaffolded locally by scripts/create-project.sh are their own repos — not tracked here
+/projects/