[t3-code-sandboxes]

Stacked on #68.

Problem. In a sandbox chat, the model picker and the model and effort it sends didn't behave like a normal T3 chat:

• While nothing had read the sandbox yet (connecting, host restarted, sandbox stopped), the picker showed "Sandbox agent" (the bridge's probe placeholder) or another sandbox chat's models.
• A send in that state dropped the chat's effort, since the composer derived options against that list, and the bridge then pushed the stripped selection to the sandbox.
• The bridge compared selections strictly, so [effort:high] and [effort:high, fastMode:false] counted as different, and a host pick that only omitted options removed them from the sandbox.
• A bridge session opened before the sandbox answered could never switch models ("The active ACP session does not expose a model-switch capability.").
• The model pick copied between the sandbox's thread and its host mirror failed silently.

Fix (sandbox code only; generic composer and provider logic are unchanged).

• Until the sandbox is read, sandboxMirrorPickerSource lists exactly the mirror's own model, read-only. Each of the mirror's options becomes a one-choice control, so a send keeps them. Web and mobile both get this through client-runtime.
• sandboxSelectionFor compares by meaning:
  • Host options win.
  • An option the host leaves out keeps the sandbox's value.
  • An option set to its default counts the same as an omitted one.
  • It returns null when the model and the effective options already match.
• The sandbox bridge flavor sets modelOptionArrivesLate, as Kiro's does, because the bridge lists the sandbox's models only once the sandbox answers.
• A failed partner write is logged with console.warn. The composer keeps the pick, so the next send carries it.

Test plan

• sandboxMirror.test.ts: offers only the mirror's own model and options while neither has read the sandbox, and the composer's helpers keep [effort:xhigh, fastMode:true] (fails on the base)
• bridge.test.ts: compares options by meaning and keeps the sandbox's for any the host leaves out (fails on the base)
• AcpRegistryAdapterV2.test.ts: a bridge session opened with no model option still reports supportsModelSwitchInSession (fails on the base)
• Typecheck of client-runtime, sandbox-bridge, provider-acp-registry, server, web and mobile. In check:changed, format, lint, knip and react compiler pass. Its test step fails only the 4 remote-parent cases, whose fixture lacks modelSelection; test: fix the tests that fail on the sandbox branch itself #69 fixes that fixture.

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