docs(local-model): label the route experimental and write down the rules

- docs/api-reference.md: GET /api/local-model is labeled experimental
  until the step that sets a model up ships, so its payload is not a
  stable /api/v1 contract one stage before that step changes it. The
  section now describes the verdict as judged, the three sentences of
  the memory reason, the notes, the speed rounding and which size to
  show as the installed memory. The os-fallback description was wrong:
  the free disk space is reported there too.
- docs/versioning-policy.md: the route is listed under the experimental
  surfaces.
- docs/wiki/Settings-Reference.md: App Settings > Models > Model on this
  Mac, in the words of a user.
- CLAUDE.md and src/web/CLAUDE.md: the invariants the code relies on
  (read-only, the probe, harness rows as data, the meaning of limited,
  the word Recommended, one unit, labels from the payload, when the
  card and the group show).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/wiki/Settings-Reference.md

@@ -193,6 +193,20 @@ attached.
 section to the Run dropdown, for pointing a harness at your own OpenAI-compatible server
 instead of its native cloud backend. See [Custom Model Endpoints](Custom-Model-Endpoints).
 
+**Model on this Mac** is not a setting: it is a read-only answer. When the Codeman server
+runs on a Mac with Apple silicon, it shows that Mac's chip, memory and free disk space, the
+local coding model that fits it best with the agent CLI to run it, what the model needs of
+the memory the Mac can give one, its download size and an estimated speed, and the other
+models that fit (Good or Tight for the memory, Slow or Speed unknown for the speed). A Mac
+that can only run a model under 12 billion parameters is told so in plain words, and a Mac
+with too little memory, too little free disk space or an Intel processor is told why nothing
+is offered. The numbers are estimates from a catalogue that ships with each release; nothing
+has been tested on your Mac. **Nothing is downloaded or installed from here yet**: setting
+the model up from Codeman is the next step and is not built. The same answer appears as a
+card on the welcome screen, which you can hide per device (it stays here). The group is not
+shown when the server runs on Linux or Windows, and in multi-user mode only an admin sees
+it. The "Mac" is always the machine the server runs on, not the device you are reading on.
+
 ### Agents & CLIs
 
 | Setting                          | Notes                                                                                        |