Floor the structured option catalog on the seed the client already holds

`structuredAgentSessionOptionCatalog` runs on the client, over a result a host of
any version published — both `use-structured-agent-session.ts` and mobile's
`use-mobile-structured-agent-options.ts` hand it straight off the
`agentSession.options` wire. It mapped `result.models` through unchanged, so an
empty list reached the snapshot, which correctly returns `[]` for an empty
catalog, and the model pill and effort pill both vanished.

On `origin/main` that was unreachable: this function unconditionally pushed
`current.model`, so the list was never empty. Withholding unofficial ids removed
that push, so the exposure is this PR's. Flooring only the host-side readers
closes it for new hosts; a new client paired with an older host still hits it,
which is the divergence `docs/reference/remote-wire-compatibility.md` covers
when a host changes what it publishes.

The floor uses `seed.models`, the catalog the client already passes in, so it
holds for Claude and any future structured agent rather than one provider. The
reader-side floors stay as defence in depth.

src/shared/structured-agent-session-options.test.ts

@@ -1,5 +1,8 @@
 import { describe, expect, it } from 'vitest'
-import { CODEX_SESSION_OPTION_CATALOG } from './agent-session-option-catalog-claude-codex'
+import {
+  CLAUDE_SESSION_OPTION_CATALOG,
+  CODEX_SESSION_OPTION_CATALOG
+} from './agent-session-option-catalog-claude-codex'
 import { buildNativeChatSessionOptionSnapshot } from './native-chat-session-option-snapshot'
 import { createNativeChatSessionOptionRecord } from './native-chat-session-option-state'
 import {
@@ -93,6 +96,33 @@ describe('structured agent session options', () => {
     })
   })
 
+  it.each([
+    { agent: 'codex' as const, seed: CODEX_SESSION_OPTION_CATALOG },
+    { agent: 'claude' as const, seed: CLAUDE_SESSION_OPTION_CATALOG }
+  ])(
+    'keeps the options row when an older host publishes an empty list: $agent',
+    ({ agent, seed }) => {
+      // Wire case, not a unit case: `structuredAgentSessionOptionCatalog` runs on the client over
+      // whatever a host published. A host that predates the readers' own seed floor still sends
+      // `models: []` beside a current model for a restored thread, and the row must survive it.
+      const state = applyStructuredAgentSessionOptions(
+        createStructuredAgentSessionOptionState(agent),
+        seed,
+        { models: [], current: { model: 'unlisted-from-an-old-host' } }
+      )
+
+      const snapshot = structuredAgentSessionOptionSnapshot(state)
+      expect(snapshot.map((descriptor) => descriptor.id)).toEqual(['model', 'effort'])
+      const model = snapshot[0]!
+      // Only official names reach the pill; the raw id is neither offered nor shown.
+      expect(model).toMatchObject({ valueSource: 'unknown' })
+      expect(model.kind.type === 'select' ? model.kind.choices.map((c) => c.value) : []).toEqual(
+        seed.models.map((seeded) => seeded.id)
+      )
+      expect(snapshot[1]).toMatchObject({ id: 'effort', settable: true, kind: { type: 'select' } })
+    }
+  )
+
   it('projects live options as directly settable descriptors', () => {
     const state = applyStructuredAgentSessionOptions(
       createStructuredAgentSessionOptionState('codex'),

src/shared/structured-agent-session-options.ts

@@ -52,7 +52,12 @@ export function structuredAgentSessionOptionCatalog(
   // Why: only ids the provider lists may be offered. An unlisted `current.model` still
   // reaches the snapshot through the record, which draws it as tracked-but-unmatched —
   // a neutral pill plus effort from `unknownModelOptions` — instead of a raw-id row.
-  const models: CatalogModel[] = result.models.map(discoveredModel)
+  const discovered: CatalogModel[] = result.models.map(discoveredModel)
+  // Why here and not only in each host-side reader: this runs on the client, on a result a host
+  // of any version published. An empty list offers nothing to pick and carries no options, so the
+  // snapshot drops the whole row — and a host that predates the readers' own seed floor publishes
+  // exactly that for a restored thread whose provider listed nothing. The seed is agent-agnostic.
+  const models = discovered.length > 0 ? discovered : [...seed.models]
   return { ...seed, models, defaultModelIsCliDefault: true }
 }