Thread the live model into the picker instead of inferring it

[TheGreatAxios]

Summary

The model picker marked "(current)" on recentModels[0] — the model
most recently switched to via /model — instead of the model the
session was actually running. A session that starts on
defaultProvider from settings and never opens the picker gets
mislabeled whenever a previous session's recents point elsewhere: the
wrong row reads (current), and the real model gets no marker at all.

Fixes CL-5597 by threading the live provider/model down through
RunnerHostDeps.activeModel → ProductHostConfig.activeModelId, read
fresh on every picker open. The recents-based guess in
product-host.ts is deleted, not kept alongside the real value.

Net diff: +63/-6 lines, mostly test coverage (two new/updated tests);
production code (product-host.ts, runner-host.ts, runner.ts) is
net smaller than before — it deletes the inference helper and adds one
line of live wiring.

Verification

• bun run typecheck — passes

• bun run build — passes

• bun run test — 4038 pass, 0 fail

• bun test ./src ./tests ./evals --randomize --seed 42 — 4038 pass, 0 fail

• Confirmed red: with the fix's activeModelId wiring reverted, the new
  product-host.test.ts case fails (xai/thegreataxios / grok-4.5 (current)
  shown instead of the live codex/abk-labs), then passes after restoring
  the fix.

• Rendered a real frame: recents hold xai/thegreataxios / grok-4.5 from a
  prior session; this session runs codex/abk-labs / gpt-5.5 and never
  opened /model. The picker now marks the live model, not the stale
  recent:

  ┌──────────────────────────────────────────────────────────────────────────┐
  │ model / provider · Esc cancel · Enter choose                             │
  │ > xai/thegreataxios / grok-4.5                                           │
  │   codex/abk-labs (current)                                               │
  │ ──────────────────────────────────────────────────────────────────────── │
  └──────────────────────────────────────────────────────────────────────────┘
  

Closes CL-5597

[linear-code]

CL-5597

[TheGreatAxios]

Independent review — ready to merge

Verified by running, not by reading. Held open for human review, not merged.

The stale-recents bug is fixed. A real rendered frame with recents holding xai/thegreataxios / grok-4.5 and the live session on codex/abk-labs / gpt-5.5, picker never opened: the stale row is not marked, and gpt-5.5 (current) renders correctly.

Degrades sanely when the live model is absent from the catalog. No test covered this, so the reviewer wrote one: an activeModelId pointing at a model not in the catalog neither throws nor marks an arbitrary row. annotateCurrent short-circuits on undefined and its equality branch simply never hits.

One path, not two. grep -rn 'section === "recent"' src/ finds only unrelated assertions about recents-list contents — nothing decides (current) anymore. The old inference helper has no surviving caller.

The optional config degrades to no marker, not to the old behavior. A host omitting activeModel/activeModelId yields undefined and renders no marker — an honest absence rather than a silent revert to the stale inference.

The threaded values are genuinely live. src/tui/runner.ts:381 declares let config, onModelSelect at :1943 reassigns it, and the activeModel closure at :1935 reads the same binding — so it observes a mid-session model switch rather than a startup snapshot.

Red/green is real. 7a6e03a (failing test) precedes 5ecfb4d (fix), and checking out the earlier commit produces a genuine failure — a TS2353 type error, because the test referenced a ProductHostConfig field production code did not yet expose.

Typecheck and build clean. 4038 pass / 0 fail, identical under --randomize --seed 42. Commit hygiene clean: three verb-first subjects, no over-length lines, no ticket IDs or filenames, no any, no vendor/ changes.

One trivial note, deliberately not sent back to the author: product-host.ts:578-579 keeps a one-line activeModelId wrapper that callers could inline. Too small to be worth a round trip.