feat(custom-model): generate Run-menu entries from saved endpoint profiles

Follow-up to #393, picking up the work Ark0N invited in his merge comment:
"generate those entries from the saved profiles rather than a fixed
duplicate per harness, and put it in a follow-up PR so this one stays the
backend... The Run-menu picker is yours if you want it."

Adds the frontend surface the backend has been waiting on:

- Run menu: a "Custom Endpoints" section lists one entry per (harness that
  supports customModelInjection, saved endpoint) pair, e.g.
  "Claude Code (llama.cpp)". The harness list comes from
  window.__codemanCustomModelClis, injected at page render straight off the
  CLI registry's own capabilities (never a hardcoded id list in the
  frontend), so a CLI whose injection recipe lands later appears with no
  frontend change. Picking an entry runs that harness's own existing run*()
  function unmodified (case creation, env overrides, everything, forced to
  a single instance) and then applies the endpoint's default model to the
  session it creates via the existing POST /api/sessions/:id/custom-model
  route. Entries are hidden for a remote/docker active case, since that
  route already refuses both.
- Settings: App Settings -> Models gets a "Custom model endpoints" group
  wiring up the customModelEndpointsEnabled toggle (declared since #393,
  read by nothing until now) plus CRUD against the existing
  /api/model-endpoints routes: list, add/edit (inline form), delete,
  discover models.
- Backend: CustomModelHost gains an optional defaultModelId, the model the
  picker applies with no further choice per endpoint (one generated menu
  entry per CLI+endpoint pair, not per CLI+endpoint+model). The route
  refuses a value that isn't one of the endpoint's own discovered models,
  and a fresh discovery drops a default that no longer appears rather than
  carrying an invalid one forward.

Docs: docs/custom-model-endpoints.md describes the new picker and settings
panel; CLAUDE.md's Custom Model Endpoint Profiles entry drops the
"backend-only" status note and documents the picker's generation mechanism.

Tests: four new route tests cover defaultModelId validation, acceptance,
and the drop/keep behaviour across a re-discovery; a new render-index-html
test pins the __codemanCustomModelClis injection (present, agent CLIs
supporting the capability, antigravity and shell excluded) and its
solo-window skip. No browser test was added for the Run-menu picker itself
or the settings CRUD panel (this box has no tmux, so the live server used
by test:browser/test:mobile could not be exercised here) -- worth a
Playwright pass before merge, same as any other frontend PR.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RqZeHrRS6DYcGcGX2p9EwG

test/render-index-html.test.ts

@@ -187,6 +187,28 @@ describe('WebServer.renderIndexHtml', () => {
     });
   });
 
+  it('reports which run modes the custom-model Run-menu picker may generate an entry for', async () => {
+    // Read generically off the CLI registry's own capabilities, not a hardcoded id
+    // list — antigravity (`unsupported`) and shell (`kind !== 'agent'`) must be
+    // absent, and any enabled agent CLI with a real injection recipe must be
+    // present, with no mock needed since this reads the real stock registry.
+    const { server } = makeServer({});
+    const html = await render(server);
+    expect(html).toContain('window.__codemanCustomModelClis=');
+    const clis = JSON.parse(html.match(/window\.__codemanCustomModelClis=(\[.*?\]);/)![1]) as Array<{
+      id: string;
+      label: string;
+    }>;
+    const ids = clis.map((c) => c.id);
+    expect(ids).toContain('claude');
+    expect(ids).not.toContain('antigravity');
+    expect(ids).not.toContain('shell');
+    for (const cli of clis) {
+      expect(typeof cli.id).toBe('string');
+      expect(typeof cli.label).toBe('string');
+    }
+  });
+
   it('still emits the object when nothing at all is installed', async () => {
     // The all-false case is the one that matters most and the easiest to get
     // wrong by only injecting when something resolves.
@@ -218,6 +240,7 @@ describe('WebServer.renderIndexHtml', () => {
     const { server } = makeServer({});
     const html = await render(server, 'sess-123');
     expect(html).not.toContain('__codemanCliAvailable');
+    expect(html).not.toContain('__codemanCustomModelClis');
   });
 
   it('does not expose gesture at all when CODEMAN_GESTURE is unset', async () => {