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>

flake.lock

@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1751274312,
+        "narHash": "sha256-/bVBlRpECLVzjV19t5KMdMFWSwKLtb5RyXdjz3LJT+g=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "50ab793786d9de88ee30ec4e4c24fb4236fc2674",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "50ab793786d9de88ee30ec4e4c24fb4236fc2674",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}