fix(runtime): give the structured-session refusal its own error code

The `agentSession.*` gate refused with a bare `Error` whose message was the
magic token, so the RPC envelope flattened it to `runtime_error` and a client
could not structurally tell "this host refuses structured chat" from any other
runtime fault. The gate now throws the repo's coded-error shape and the code is
registered for envelope passthrough; the message is unchanged, so clients that
match on the token keep classifying exactly as before.

src/main/runtime/rpc/errors.ts

@@ -109,6 +109,9 @@ const STRUCTURED_RUNTIME_PASSTHROUGH_CODES: ReadonlySet<string> = new Set([
   'runtime_timeout',
   'invalid_runtime_response',
   'capability_unsupported',
+  // The whole `agentSession.*` gate refusal. Coded so a client can route on it without matching
+  // the message; the message is unchanged so older clients still classify.
+  'structured_agent_session_unsupported',
   'relay_quota_exceeded',
   'dispatch_capability_invalid',
   'agent_unconfigured',

src/main/runtime/rpc/methods/structured-agent-session-gate.ts

@@ -8,6 +8,7 @@
 // for old mobile clients while structured chat is enabled so they receive a fallback row, and that
 // path constructs the host. `agentSession.*` stays refused either way, which is what this gate is for.
 
+import { OrchestrationError } from '../../orchestration/orchestration-error'
 import { getStructuredAgentSessionHost } from '../../../native-chat/agent-session-wire/structured-agent-session-registry'
 import type { StructuredAgentSessionHost } from '../../../native-chat/agent-session-wire/structured-agent-session-host'
 import type { StructuredAgentSessionCaller } from '../../../native-chat/agent-session-wire/structured-agent-session-host-types'
@@ -17,6 +18,17 @@ import {
   supportsStructuredAgentSessions
 } from './structured-agent-session-policy'
 
+/**
+ * Carries a code so a client can tell "this host refuses structured chat" apart from any other
+ * runtime fault. The message stays the bare token: older clients classify by matching it.
+ */
+function structuredSessionsUnsupported(): OrchestrationError {
+  return new OrchestrationError(
+    'structured_agent_session_unsupported',
+    'structured_agent_session_unsupported'
+  )
+}
+
 /**
  * In-process callers are the same build as the host, so they carry no negotiated
  * capability list; every remote client must say it can read structured sessions.
@@ -27,7 +39,7 @@ export function supportsStructuredSessions(ctx: RpcContext): boolean {
 
 export function requireStructuredCapability(ctx: RpcContext): void {
   if (!supportsStructuredSessions(ctx)) {
-    throw new Error('structured_agent_session_unsupported')
+    throw structuredSessionsUnsupported()
   }
 }
 
@@ -35,7 +47,7 @@ export function requireStructuredHost(ctx: RpcContext): StructuredAgentSessionHo
   requireStructuredCapability(ctx)
   const host = getStructuredAgentSessionHost()
   if (!host) {
-    throw new Error('structured_agent_session_unsupported')
+    throw structuredSessionsUnsupported()
   }
   return host
 }
@@ -64,11 +76,11 @@ export function requireStructuredHost(ctx: RpcContext): StructuredAgentSessionHo
  */
 export function requireStructuredCleanupHost(ctx: RpcContext): StructuredAgentSessionHost {
   if (!supportsStructuredAgentSessionCapability(ctx)) {
-    throw new Error('structured_agent_session_unsupported')
+    throw structuredSessionsUnsupported()
   }
   const host = getStructuredAgentSessionHost()
   if (!host) {
-    throw new Error('structured_agent_session_unsupported')
+    throw structuredSessionsUnsupported()
   }
   return host
 }

src/main/runtime/rpc/methods/structured-agent-session.test.ts

@@ -177,6 +177,20 @@ describe('capability gating', () => {
     expect(hostCalls.send).not.toHaveBeenCalled()
   })
 
+  it('refuses with a code a client can route on, not just a message it must parse', async () => {
+    const response = await call('agentSession.send', sendParams(), {
+      clientKind: 'runtime',
+      clientCapabilities: ['terminal.stream.v1']
+    })
+    expect(response).toMatchObject({
+      ok: false,
+      error: {
+        code: 'structured_agent_session_unsupported',
+        message: 'structured_agent_session_unsupported'
+      }
+    })
+  })
+
   it('rejects create intent before resolving host-owned fields for an old client', async () => {
     const worktree = 'id:workspace-1'
     const response = await call(