Add header status pill showing task/session/sandbox state

Surface aggregate status (error, provisioning, running, ready, no sandbox)
as a colored pill in the transcript panel header. Integrates task runtime
status, session status, and sandbox availability via the sandboxProcesses
interest topic so the pill accurately reflects unreachable sandboxes.

Includes mock tasks demonstrating error, provisioning, and running states,
unit tests for deriveHeaderStatus, and workspace-dashboard integration.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

foundry/packages/backend/test/helpers/test-driver.ts

@@ -1,23 +1,10 @@
-import type {
-  BackendDriver,
-  DaytonaClientLike,
-  DaytonaDriver,
-  GitDriver,
-  GithubDriver,
-  StackDriver,
-  SandboxAgentDriver,
-  SandboxAgentClientLike,
-  TmuxDriver,
-} from "../../src/driver.js";
-import type { ListEventsRequest, ListPage, ListPageRequest, ProcessInfo, ProcessLogsResponse, SessionEvent, SessionRecord } from "sandbox-agent";
+import type { BackendDriver, GitDriver, GithubDriver, StackDriver, TmuxDriver } from "../../src/driver.js";
 
 export function createTestDriver(overrides?: Partial<BackendDriver>): BackendDriver {
   return {
     git: overrides?.git ?? createTestGitDriver(),
     stack: overrides?.stack ?? createTestStackDriver(),
     github: overrides?.github ?? createTestGithubDriver(),
-    sandboxAgent: overrides?.sandboxAgent ?? createTestSandboxAgentDriver(),
-    daytona: overrides?.daytona ?? createTestDaytonaDriver(),
     tmux: overrides?.tmux ?? createTestTmuxDriver(),
   };
 }
@@ -63,79 +50,6 @@ export function createTestGithubDriver(overrides?: Partial<GithubDriver>): Githu
   };
 }
 
-export function createTestSandboxAgentDriver(overrides?: Partial<SandboxAgentDriver>): SandboxAgentDriver {
-  return {
-    createClient: (_opts) => createTestSandboxAgentClient(),
-    ...overrides,
-  };
-}
-
-export function createTestSandboxAgentClient(overrides?: Partial<SandboxAgentClientLike>): SandboxAgentClientLike {
-  const defaultProcess: ProcessInfo = {
-    id: "process-1",
-    command: "bash",
-    args: ["-lc", "echo test"],
-    createdAtMs: Date.now(),
-    cwd: "/workspace",
-    exitCode: null,
-    exitedAtMs: null,
-    interactive: true,
-    pid: 123,
-    status: "running",
-    tty: true,
-  };
-  const defaultLogs: ProcessLogsResponse = {
-    processId: defaultProcess.id,
-    stream: "combined",
-    entries: [],
-  };
-  return {
-    createSession: async (_prompt) => ({ id: "test-session-1", status: "running" }),
-    sessionStatus: async (sessionId) => ({ id: sessionId, status: "running" }),
-    listSessions: async (_request?: ListPageRequest): Promise<ListPage<SessionRecord>> => ({
-      items: [],
-      nextCursor: undefined,
-    }),
-    listEvents: async (_request: ListEventsRequest): Promise<ListPage<SessionEvent>> => ({
-      items: [],
-      nextCursor: undefined,
-    }),
-    createProcess: async () => defaultProcess,
-    listProcesses: async () => ({ processes: [defaultProcess] }),
-    getProcessLogs: async () => defaultLogs,
-    stopProcess: async () => ({ ...defaultProcess, status: "exited", exitCode: 0, exitedAtMs: Date.now() }),
-    killProcess: async () => ({ ...defaultProcess, status: "exited", exitCode: 137, exitedAtMs: Date.now() }),
-    deleteProcess: async () => {},
-    sendPrompt: async (_request) => {},
-    cancelSession: async (_sessionId) => {},
-    destroySession: async (_sessionId) => {},
-    ...overrides,
-  };
-}
-
-export function createTestDaytonaDriver(overrides?: Partial<DaytonaDriver>): DaytonaDriver {
-  return {
-    createClient: (_opts) => createTestDaytonaClient(),
-    ...overrides,
-  };
-}
-
-export function createTestDaytonaClient(overrides?: Partial<DaytonaClientLike>): DaytonaClientLike {
-  return {
-    createSandbox: async () => ({ id: "sandbox-test-1", state: "started" }),
-    getSandbox: async (sandboxId) => ({ id: sandboxId, state: "started" }),
-    startSandbox: async () => {},
-    stopSandbox: async () => {},
-    deleteSandbox: async () => {},
-    executeCommand: async () => ({ exitCode: 0, result: "" }),
-    getPreviewEndpoint: async (sandboxId, port) => ({
-      url: `https://preview.example/sandbox/${sandboxId}/port/${port}`,
-      token: "preview-token",
-    }),
-    ...overrides,
-  };
-}
-
 export function createTestTmuxDriver(overrides?: Partial<TmuxDriver>): TmuxDriver {
   return {
     setWindowStatus: () => 0,