feat(custom-model): ask which model on launch when an endpoint has more than one, and re-discover models every 5 minutes

Two enhancements requested after live-validating PR #430 against a real
llama.cpp server:

1. Model picker dialog. Picking a Run-menu Custom Endpoints entry used to
   apply the endpoint's defaultModelId (or the first discovered model)
   silently. Now, via the new selectCustomModelEntry() (session-ui.js):
   - exactly one discovered model launches straight away, same as before
   - two or more open a new #customModelPickModal listing every discovered
     model; defaultModelId (if set) is marked but never auto-chosen, since
     the point of asking is letting ONE launch deliberately differ from
     the saved default, not just confirming it
   The endpoint is re-fetched at click time rather than trusting anything
   cached from the dropdown's own render, since the model list can have
   changed (the sweep below, or a settings-panel edit) since it opened.
   runCustomModelEntry() itself — the actual launch, routed through run()
   for the in-flight lock, snapshot-guarded against applying to the wrong
   session — is unchanged; it now just always receives an explicit model
   id from one of these two paths instead of computing one itself.

2. Periodic re-discovery. Every saved endpoint's models now refresh
   automatically every 5 minutes in the background
   (CUSTOM_MODEL_REDISCOVER_INTERVAL_MS, server.ts, registered the same way
   as the Codex plan-usage poll it sits beside — this.cleanup.setInterval,
   off under testMode), so a model the server starts or stops serving shows
   up without another manual "Discover" click. The manual POST
   .../discover-models route and the new refreshAllCustomModelHosts()
   sweep (custom-model-routes.ts) now share one pure merge step
   (applyDiscoveredModels: stamps lastDiscoveredAt, drops a defaultModelId
   that no longer appears) rather than two copies that could drift. The
   sweep is best-effort per host — one endpoint being unreachable on a
   cycle never blocks the others — and re-reads the store before each
   host's write, keyed by id, so a concurrent edit or delete from the
   settings panel always wins over a sweep that started before it.

Tests: test/custom-model-endpoint-rediscovery.test.ts is a new, dedicated
file for the sweep (kept separate from custom-model-routes.test.ts because
that file's data dir is shared across every test in it — one temp HOME per
FILE, not per test — which would make a sweep-touches-every-host assertion
meaningless there). test/custom-model-run-menu-ui.test.ts gained a new
describe block driving the real picker modal through JSDOM: single-model
bypass, multi-model dialog with the default marked-not-chosen, picking a
row closes the modal and launches with that exact model, the endpoint
re-fetch, and the two "vanished by click time" toast paths.

Docs: docs/custom-model-endpoints.md, docs/wiki/Custom-Model-Endpoints.md,
docs/api-reference.md and CLAUDE.md's dense feature paragraph all updated
— the last of these also caught up two sentences that had gone stale after
the draft-review fixes landed (the picker routes through run() now, not a
raw run*() call).

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

docs/custom-model-endpoints.md

@@ -66,18 +66,26 @@ configured, `PUT`/`DELETE /api/model-endpoints/:id` update or remove one.
 Endpoint management is admin-only in multi-user mode, same as remote/docker
 hosts — these are machine-level infra, not per-user settings.
 
-`defaultModelId` names which discovered model the Run-menu picker applies
-for that endpoint with no further choice — the settings panel's Edit form
-exposes it as a select populated from the endpoint's own discovered
-`models`, and the route refuses a value that isn't one of them. Leaving it
-unset falls back to the first discovered model; re-discovering drops a
-default that no longer appears in the fresh list rather than carrying an
-invalid one forward.
+`defaultModelId` names which discovered model the picker pre-marks for that
+endpoint — the settings panel's Edit form exposes it as a select populated
+from the endpoint's own discovered `models`, and the route refuses a value
+that isn't one of them. It is applied automatically only when the endpoint
+has exactly one discovered model (nothing to choose); with two or more it
+is a pre-selection in the model-picker dialog below, never a silent default.
+Re-discovering drops a default that no longer appears in the fresh list
+rather than carrying an invalid one forward.
+
+**Model lists refresh themselves.** A background sweep (`server.ts`,
+`CUSTOM_MODEL_REDISCOVER_INTERVAL_MS`, every 5 minutes) re-discovers every
+saved endpoint the same way the manual `POST .../discover-models` route
+does, best-effort per endpoint — one being unreachable on a given cycle
+never blocks the others. Off under `npm test`, same reasoning as the Codex
+plan-usage poll it sits beside: no real network to hit, no server instance
+to keep the timer alive for.
 
 ## The Run-menu picker
 
-With the setting on and at least one endpoint carrying a usable default
-model (either an explicit `defaultModelId` or just one discovered model),
+With the setting on and at least one endpoint carrying a discovered model,
 the toolbar's Run dropdown grows a **Custom Endpoints** section: one entry
 per (harness that can redirect to a custom endpoint, saved endpoint) pair,
 e.g. "Claude Code (llama.cpp)". The harness list is read off the CLI
@@ -86,13 +94,22 @@ registry's own `capabilities.customModelInjection` at page render
 in the frontend — so a CLI whose injection recipe lands later shows up with
 no frontend change, and Antigravity (`unsupported`) never does.
 
-Picking an entry runs a single session on that harness exactly the way its
+Picking an entry re-fetches the endpoint (`selectCustomModelEntry()`,
+`session-ui.js`) rather than trusting anything cached from the dropdown's
+own render — the model list can have changed via the 5-minute sweep above
+or a settings-panel edit since the menu opened. With exactly one discovered
+model it runs straight away; with two or more, a small modal
+(`#customModelPickModal`) lists them and asks which one to use for this
+launch, with the endpoint's `defaultModelId` marked but not auto-chosen —
+the point of asking is letting one launch deliberately differ from the
+saved default, not just confirming it. Whichever way the model was decided,
+the launch itself runs a single session on that harness exactly the way its
 own Run-menu entry would (same case creation, env overrides, everything),
-then immediately applies the endpoint's default model to it via the route
-below. It is a one-off "try this endpoint" action, not a sticky mode: the
-plain Run button still means "this harness, native cloud" afterward.
-Entries are hidden entirely for a remote or Docker active case, since the
-apply route refuses both (see the next section).
+then immediately applies the endpoint and model to it via the route below.
+It is a one-off "try this endpoint" action, not a sticky mode: the plain
+Run button still means "this harness, native cloud" afterward. Entries are
+hidden entirely for a remote or Docker active case, since the apply route
+refuses both (see the next section).
 
 ## Applying a model to a session