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/README.md

@@ -0,0 +1,46 @@
+# Headless automation API
+
+Open CAD Studio can run without a GUI and be driven over a line-based JSON
+protocol — for scripts, batch jobs, or AI agents.
+
+```sh
+OpenCADStudio --serve
+```
+
+It reads one JSON request per line on **stdin** and writes one JSON response per
+line on **stdout**. The active document persists across requests, so a caller
+can act → observe → act.
+
+## Protocol
+
+| Request | Response |
+|---------|----------|
+| `{"op":"new"}` | `{"ok":true,"total":0,"by_type":{}}` |
+| `{"op":"open","path":"plan.dwg"}` | entity summary |
+| `{"op":"run","cmd":"LAYER Walls"}` | `{"ok":true,"cmd":...,"entities":N,"added":D}` |
+| `{"op":"entities"}` | `{"ok":true,"total":N,"by_type":{"Line":42,...}}` |
+| `{"op":"save","path":"out.dwg"}` | `{"ok":true,"saved":"out.dwg"}` (path optional once opened/saved) |
+
+Every response has `"ok"`; failures carry `"error"`. `run` drives Open CAD
+Studio's **own** command system — no separate bindings to maintain — so its
+coverage grows with the app.
+
+> **Status (first increment):** `run` applies synchronous commands (system
+> variables, layer ops, …). Pick-based interactive commands (drawing by clicking
+> points) need coordinate feeding and are not wired headless yet.
+
+## Python client
+
+[`ocs.py`](ocs.py) is a ~100-line client — nothing to compile:
+
+```python
+from ocs import Ocs
+
+with Ocs(binary="OpenCADStudio") as ocs:   # spawns `--serve`
+    ocs.open("plan.dwg")
+    ocs.run("LAYER Walls")
+    print(ocs.entities())
+    ocs.save("plan_out.dwg")
+```
+
+Any language can speak the same protocol over a subprocess pipe.