feat: new settings, CLI skeleton and interface skeletons

Adds the pydantic-settings Settings with validated GITEA_URL, OAuth scopes
and toolsets, the argparse CLI skeleton, catalog.model, identity and
GiteaAPIError. Test fixtures now isolate env, cwd and state paths in tmp.

src/aegis_gitea_mcp/cli.py

@@ -0,0 +1,55 @@
+"""Command line entry point (``aegis-gitea-mcp``).
+
+Subcommands: ``serve`` (default), ``setup`` and ``version``. ``serve`` and
+``setup`` are filled in by later chunks.
+"""
+
+from __future__ import annotations
+
+import argparse
+import sys
+from collections.abc import Sequence
+
+from aegis_gitea_mcp import __version__
+
+
+def build_parser() -> argparse.ArgumentParser:
+    """Build the argument parser."""
+    parser = argparse.ArgumentParser(
+        prog="aegis-gitea-mcp",
+        description="MCP server exposing the Gitea API over stdio or streamable HTTP.",
+    )
+    subparsers = parser.add_subparsers(dest="command")
+
+    serve = subparsers.add_parser("serve", help="run the MCP server (default)")
+    serve.add_argument(
+        "--transport",
+        choices=("stdio", "http"),
+        default=None,
+        help="transport to serve (default: MCP_TRANSPORT, else stdio)",
+    )
+
+    subparsers.add_parser("setup", help="interactive setup against a Gitea instance")
+    subparsers.add_parser("version", help="print the version and exit")
+    return parser
+
+
+def _not_implemented(command: str) -> int:
+    """Report an unbuilt command and return the usage-error exit code."""
+    print(f"aegis-gitea-mcp {command}: not implemented yet", file=sys.stderr)
+    return 2
+
+
+def main(argv: Sequence[str] | None = None) -> int:
+    """Run the CLI and return the process exit code."""
+    args = build_parser().parse_args(argv)
+    command = args.command or "serve"
+
+    if command == "version":
+        print(__version__)
+        return 0
+    return _not_implemented(command)
+
+
+if __name__ == "__main__":  # pragma: no cover
+    raise SystemExit(main())