feat(P01): nova/cli.py entry point + dispatch + audit (REQ-324, INV-12, cli-engineer)

- main(argv) builds top-level argparse(prog="nova") with required subparsers.
- Auto-discovers nova/<module>.py via pkgutil.iter_modules(nova.__path__),
  skipping `cli`; each module exports add_parser(subparsers) + run(args) -> int.
- Before dispatch: resolve_mode_from_env() → emit cli.invocation audit
  event (INV-12) as a stderr JSON line stub with mode, selection_reason,
  credential_type, command, args. Real outbox wiring comes later.
- Dispatch: args._run(args); exit code via sys.exit(main()).
- nova/__init__.py empty package marker.

---ci---
project: acdl
phase: 1
milestone: v1.28
status: execute
persona: cli-engineer
---/ci---

nova/__init__.py

@@ -0,0 +1 @@
+"""Nova CLI package — thin subcommand delegates to core.* (P1, REQ-324)."""

nova/cli.py

@@ -0,0 +1,60 @@
+"""Nova CLI entry point — dispatch + audit (P1, REQ-324, INV-12).
+
+Auto-discovers nova/<module>.py subcommands; each exports
+add_parser(subparsers) + run(args) -> int. Resolves the client mode
+via core.mode_resolver and emits a cli.invocation audit event (stderr
+JSON line stub) before dispatching.
+"""
+
+from __future__ import annotations
+
+import argparse
+import importlib
+import json
+import pkgutil
+import sys
+from typing import Optional
+
+from core.mode_resolver import resolve_mode_from_env
+
+
+def _emit_invocation(mode, reason, cred_type, command, args):
+    """INV-12: emit cli.invocation audit event to stderr (stub)."""
+    event = {
+        "event": "cli.invocation",
+        "mode": mode,
+        "selection_reason": reason,
+        "credential_type": cred_type,
+        "command": command,
+        "args": args,
+    }
+    sys.stderr.write(json.dumps(event, sort_keys=True) + "\n")
+
+
+import nova
+
+
+def _build_parser():
+    parser = argparse.ArgumentParser(prog="nova", description="Nova platform CLI")
+    parser.add_argument("--mode", choices=["agent", "interactive"], default=None)
+    sub = parser.add_subparsers(dest="command", required=True)
+    for mod_info in pkgutil.iter_modules(nova.__path__):
+        name = mod_info.name
+        if name == "cli":
+            continue
+        mod = importlib.import_module(f"nova.{name}")
+        mod.add_parser(sub)
+    return parser
+
+
+def main(argv: Optional[list] = None) -> int:
+    parser = _build_parser()
+    args = parser.parse_args(argv)
+    mode, reason = resolve_mode_from_env()
+    arg_dict = {k: v for k, v in vars(args).items() if k != "_run"}
+    _emit_invocation(mode, reason, None, args.command, arg_dict)
+    return args._run(args)
+
+
+if __name__ == "__main__":
+    sys.exit(main())