feat(gui): host goal handlers and slash command dispatch on web prompts

- web host boots CommandService + command-goal; session.prompt intercepts
  a lone text block starting with '/' and executes it as a command
  (unknown -> RPC unknown-command, usage error -> RPC command-error, so
  the composer restores the draft and shows the error strip) instead of
  sending it to the model — symmetric with the ACP adapter
- prompt response carries command result text on the wire for future UI
- api-proxy also lands the host-side goals RPC handlers

packages/host/apiproxy/tests/rpc-schemas.spec.ts

@@ -31,11 +31,14 @@ describe('rpcErrorSchema', () => {
     expect(rpcErrorSchema.parse({ code: 'bad-request', message: 'm', details: { issues: [] } }).code).toBe('bad-request')
     expect(rpcErrorSchema.parse({ code: 'session-not-found', message: 'm', details: { sessionId: 's' } }).code).toBe('session-not-found')
     expect(rpcErrorSchema.parse({ code: 'agent-busy', message: 'm', details: { reason: 'r' } }).code).toBe('agent-busy')
+    expect(rpcErrorSchema.parse({ code: 'command-error', message: 'm', details: {} }).code).toBe('command-error')
+    expect(rpcErrorSchema.parse({ code: 'unknown-command', message: 'm', details: {} }).code).toBe('unknown-command')
     expect(rpcErrorSchema.parse({ code: 'internal', message: 'm', details: {} }).code).toBe('internal')
   })
 
   it('rejects a known code with missing details', () => {
     expect(() => rpcErrorSchema.parse({ code: 'agent-busy', message: 'm', details: {} })).toThrow()
+    expect(() => rpcErrorSchema.parse({ code: 'command-error', message: 'm' })).toThrow()
     expect(() => rpcErrorSchema.parse({ code: 'nope', message: 'm', details: {} })).toThrow()
   })
 })
@@ -103,6 +106,11 @@ describe('sessions domain schemas', () => {
     expect(prompt.mode).toBe('queue')
     expect(() => sessionPromptRequestSchema.parse({ sessionId: 's1', mode: 'inject', content: [] })).toThrow()
     expect(sessionPromptValueSchema.parse({ accepted: true }).accepted).toBe(true)
+    // The command slot appears only when the prompt dispatched a slash command.
+    const dispatched = sessionPromptValueSchema.parse({ accepted: true, command: { kind: 'success', text: 'Goal set' } })
+    expect(dispatched.command?.text).toBe('Goal set')
+    expect(sessionPromptValueSchema.parse({ accepted: true, command: { kind: 'success' } }).command).toEqual({ kind: 'success' })
+    expect(() => sessionPromptValueSchema.parse({ accepted: true, command: { kind: 'failure' } })).toThrow()
     expect(sessionCancelRequestSchema.parse({ sessionId: 's1' }).sessionId).toBe('s1')
     expect(sessionCancelValueSchema.parse({ accepted: true }).accepted).toBe(true)
     expect(contentBlockSchema.parse({ type: 'text', text: 'x', extra: 1 })).toMatchObject({ extra: 1 })