docs(automation): document the native entry points and their limits

Refs #1349. The automation README describes `--mcp` and never mentions
`--serve`, even though every native build accepts it and it is the only channel
that works without a display. It also leaves out the two startup dialogs, the
`document_id` routing, and what `waiting_input` actually means.

`docs/automation/native.md` covers, all measured on a Linux headless machine
with `OpenCADStudio 2026.37+165`:

- choosing between `--mcp` and `--serve`, and what each needs (display,
  no-display, `--export` for one-shot conversion);
- the writable `HOME`/`XDG_CONFIG_HOME` requirement, without which
  `ocs_sessions` answers `No user configuration directory`;
- the startup dialogs: `AssocPrompt` then `DonationPrompt`, `new` answering
  `ok` with no document until `close_modal` clears them, and that the donation
  prompt appears once per application version per profile;
- targeting a document: reads and writes follow the active tab, so
  `document_id` has to be passed explicitly after `new`;
- feeding a whole line: `waiting_input` means more input is required (not that
  nothing happened), `cancel` closes a command, and the `unconsumed` /
  `blocked_by` fields added in #1392;
- the interactive `start`/`input` shapes, including that a typed value goes
  through `kind: "text"` while a keyword uses `kind: "token"` with its payload
  in a field named `text`, and that the `TEXT` content step belongs to the
  in-place editor (`text_input` / `text_commit`);
- what only native builds have: `measure`, `ocs_capture` (inline base64, or a
  PNG path), and `set_properties` with `collection` and colours as
  `{"Index": n}` / `"ByLayer"`;
- the remaining gap: `MTEXT`, `HATCH` and `POLYGON` report `completed` with
  `added: 0` on the `--serve` path (#1390).

The README gains one line pointing to the page, placed after the tool list
rather than at the end, so it does not overlap #1394.

Docs only; no code changes.

docs/automation/README.md

@@ -17,6 +17,10 @@ The server provides four tools:
 - `ocs_execute` performs one operation, an atomic record update, or a sequential batch against the real editor.
 - `ocs_capture` returns a bounded PNG of the drawing viewport or complete window.
 
+Native builds also accept a second, headless entry point (`OpenCADStudio --serve`, one
+JSON request per line) for scripts and CI; the two native channels, their startup dialogs
+and their limits are described in [native.md](native.md).
+
 The normal flow is to call `ocs_sessions`, pass its returned `session_id` as `ocs_session_id` to the other tools, read the chosen session with `ocs_read`, then pass the returned document state into `ocs_execute`. For example, an undo request has this shape:
 
 ```json