feat(automation): headless JSON server (--serve) + Python client

Add an external automation API (issue #29 / #100 track 2): `OpenCADStudio
--serve` runs without a GUI and is driven over a line-based JSON protocol
on stdin/stdout — open / new / run / entities / save. State persists
across requests so a script or AI agent can act, observe, and act again.

`run` drives the app's existing command system rather than a separate
binding, so coverage grows with the app; synchronous commands apply now,
pick-based interactive ones come once coordinate feeding is wired. Ships
a ~100-line example ocs.py client over the same protocol — no FFI to
maintain. Headless works because the app already constructs and dispatches
GUI-less in tests.

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

docs/automation/ocs.py

@@ -0,0 +1,90 @@
+"""Thin Python client for the Open CAD Studio headless automation server.
+
+Launches `OpenCADStudio --serve` and talks to it over a line-based JSON protocol
+(one request object per line on stdin, one response per line on stdout). There
+is nothing to compile or maintain on the Python side — every method is one JSON
+message; the real work is Open CAD Studio's own command system.
+
+    from ocs import Ocs
+
+    with Ocs(binary="OpenCADStudio") as ocs:
+        ocs.open("plan.dwg")
+        ocs.run("LAYER Walls")
+        print(ocs.entities())          # {"total": 42, "by_type": {...}}
+        ocs.save("plan_out.dwg")
+
+Each call returns the parsed response dict and raises `OcsError` on `ok: false`.
+"""
+
+from __future__ import annotations
+
+import json
+import subprocess
+from typing import Any, Optional
+
+
+class OcsError(RuntimeError):
+    """Raised when the server replies with `{"ok": false, ...}`."""
+
+
+class Ocs:
+    def __init__(self, binary: str = "OpenCADStudio") -> None:
+        self.proc = subprocess.Popen(
+            [binary, "--serve"],
+            stdin=subprocess.PIPE,
+            stdout=subprocess.PIPE,
+            text=True,
+            bufsize=1,
+        )
+        self._read()  # the {"ready": true} greeting
+
+    # ── protocol ────────────────────────────────────────────────────────────
+    def _read(self) -> dict[str, Any]:
+        line = self.proc.stdout.readline()
+        if not line:
+            raise OcsError("server closed the connection")
+        return json.loads(line)
+
+    def _send(self, **req: Any) -> dict[str, Any]:
+        self.proc.stdin.write(json.dumps(req) + "\n")
+        self.proc.stdin.flush()
+        resp = self._read()
+        if not resp.get("ok", False):
+            raise OcsError(resp.get("error", "unknown error"))
+        return resp
+
+    # ── operations ──────────────────────────────────────────────────────────
+    def new(self) -> dict[str, Any]:
+        """Start an empty document."""
+        return self._send(op="new")
+
+    def open(self, path: str) -> dict[str, Any]:
+        """Load a DWG/DXF drawing."""
+        return self._send(op="open", path=path)
+
+    def run(self, cmd: str) -> dict[str, Any]:
+        """Run a command through Open CAD Studio's command system."""
+        return self._send(op="run", cmd=cmd)
+
+    def entities(self) -> dict[str, Any]:
+        """Total entity count and a breakdown by type."""
+        return self._send(op="entities")
+
+    def save(self, path: Optional[str] = None) -> dict[str, Any]:
+        """Write the document (defaults to the opened/last-saved path)."""
+        return self._send(op="save", path=path)
+
+    # ── lifecycle ───────────────────────────────────────────────────────────
+    def close(self) -> None:
+        if self.proc.stdin:
+            self.proc.stdin.close()
+        try:
+            self.proc.wait(timeout=5)
+        except subprocess.TimeoutExpired:
+            self.proc.kill()
+
+    def __enter__(self) -> "Ocs":
+        return self
+
+    def __exit__(self, *_exc: object) -> None:
+        self.close()