Structured launch availability per registered agent, with legacy-chat fallback on decline #78

Summary

Add a host-side, per-registration launch-availability gate for structured agents and make every declined structured launch land on the surface it gets today (legacy native chat for transcript-backed agents). Provider-neutral prerequisite for registering OMP without changing anyone's default.

Context

• 44madfire/orca#20 Phase A: OMP is registered as a structured provider while unsupported builds/environments stay on the old path, gated by capability/version.
• agentSession.agents publishes every registration statically (src/main/native-chat/agent-session-wire/structured-agent-session-host.ts:211 @76d1b7b); the same list bounds which saved chats a client renders (comment at :213), so dropping an agent from it would hide existing tabs.
• The renderer routes any listed agent structured (src/shared/structured-native-chat-launch-route.ts:102-106 @76d1b7b). There is no per-agent opt-in or version gate.
• Host create-support already consults a per-registration hook: structuredAgentRuntimeRegistration(agent)?.supportsLocation(location) (src/main/runtime/orca-runtime-get-structured-agent-session-create-support.ts:52 @76d1b7b); agent.launch and orchestration read it (src/main/agent-launch/agent-launch-mode.ts:192-222 @76d1b7b).
• A local structured launch opens immediately with no decline path (src/renderer/src/lib/launch-agent-in-new-tab-structured-route.ts:36-58 @76d1b7b); a paired decline opens terminal-tui (:72-75), not the legacy chat today's OMP launch gets.
• Structured children on Windows need process start-time proof (src/main/provider-process/supervised-provider-child-location.ts:9,14 @76d1b7b); WSL/SSH are blocked at structured-native-chat-launch-route.ts:97-99,111-113.
• Remote wire compatibility (docs/reference/remote-wire-compatibility.md): a new optional field is safe; a new opcode is not.

Scope

• Optional async launchAvailability on structured agent registrations, evaluated on the execution host, cached with single-flight; absent hook = available (Grok and built-ins unchanged).
• getStructuredAgentSessionCreateSupport refuses with a named reason when availability says no.
• agentSession.agents entries gain optional launchable: false and a reason; renderer hostStructuredAgents excludes non-launchable agents while their existing tabs still render and close.
• Declines (paired host decline, local availability refusal) route to what decideInitialAgentTabViewMode would choose without the structured preference (legacy native chat when the agent supports it), not hard-coded terminal-tui.
• ACP registrations report non-launchable on Windows hosts without process start-time proof.

Non-goals

• No OMP registration (Register omp acp as an opt-in structured ACP agent behind the availability gate #68). No change to the structured-chat preference settings. No new stream opcodes. No SSH/WSL structured support.

Depends on

• None.

Parity rows owned

• P-ROPT-01, P-ROPT-02, P-ROPT-03, P-ROPT-04, P-ROPT-05, P-ROPT-06, P-ROPT-07.

Acceptance criteria

• Grok and built-in agents route exactly as before on every surface.
• A non-launchable agent is never chosen structured by desktop, paired desktop, agent.launch, or orchestration; its existing structured tabs still render and close.
• Old clients ignore the new field; new clients treat a missing field as launchable.
• A declined structured launch opens legacy native chat when the agent supports it and the user's default is chat; terminal otherwise.
• Windows without start-time proof reports ACP agents non-launchable with a reason.
• Routing outcomes for desktop local, WSL, plain SSH, paired runtime, runtime-owned SSH target, Windows without proof, and folder workspaces (inside root / custom start dir) are pinned for a non-launchable agent and unchanged for launchable ones.

Required tests

• Availability cache: single in-flight probe under concurrent reads; invalidation on binary change; probe timeout → non-launchable, no hang.
• Mixed versions: old client + new host, new client + old host (field absent), paired client whose host declines after a provisional tab → exactly one surface opens.
• Probes run on the execution host only; an SSH or paired workspace never probes on the client (SSH boundary rule).
• Routing matrix table test over the seven P-ROPT rows.

Base branch

acp-migration (44madfire/orca).