[Story 3.2b] Fan-out: parent stays active while child runs #369

Part of #358 (Epic 3: TaskScheduler + Subtask Fan-out).

Depends on: Story 3.2a (TaskRegistry), Story 3.1 (TaskScheduler).

Relates to #126 (concurrent conversation sidebar — this story implements the fan-out code path that makes concurrent tasks possible).

Context

delegateParentAndOpenChild always calls removeClineFromStack (parent suspends). For fan-out, the parent should keep running while the child executes concurrently. This story adds the second code path in ClineProvider/task delegation: only when maxConcurrency > 1 and a scheduler permit can be reserved immediately, the parent remains live in TaskRegistry while the child is created and focused.

This story is also the correctness baseline for fan-out. It is not enough for both tasks to be present in TaskRegistry: the parent must continue with its own execution context while the child is being initialized. Historically getCurrentTask() was a safe implicit parameter because only one task could be alive; fan-out creates a window where getCurrentTask() still returns the parent while setup operations are logically about the child.

Developer Notes

In src/core/task/TaskScheduler.ts / src/utils/TaskSemaphore.ts:

• Expose maxConcurrency and available capacity.
• Add a non-queueing reservation path (tryReserve() / tryAcquire()) and run a task with a previously reserved permit (runWithReservation()).
• Do not decide fan-out via available > 0 followed by awaits before scheduling. Reserve the permit at the decision point.
• Every reserved permit must be released on all failure paths before the child starts running.

In src/core/webview/ClineProvider.ts:

• When fan-out path is taken: keep the parent in the registry, create/focus the child, and expose all running tasks via getRunningTasks().
• getCurrentTask() remains the UI-focused task. Do not assume it is the task an operation is logically about during child setup.
• Thread explicit task scope through fan-out-reachable operations. At minimum, the parent must not be mutated by the child's setup path:
  • parent _taskMode / task-history mode must not be overwritten by child mode
  • parent live API handler must not be rebuilt to the child's saved provider/model
  • parent sticky provider profile must not be overwritten by child profile
  • parent clineMessages / taskId must not be posted to the webview as if it were the newly focused child during the pre-child-registration window
• Protect against the broader execution-context leak: if child mode/profile activation changes global provider state that a still-running parent can read later (for example system prompt generation or provider settings reads), snapshot or explicitly scope the parent execution context so parent follow-up requests continue using the parent's mode/profile/config.
• If parent eviction happens before child creation in the non-fan-out path, any createTask() failure must restore the parent.
• If a permit is reserved in the fan-out path, any createTask() or metadata-persistence failure before child start must release the permit.
• Rollback cleanup must remove the just-created child by taskId, not only if it is currently focused.

Files: src/core/task/TaskScheduler.ts, src/utils/TaskSemaphore.ts, src/core/webview/ClineProvider.ts

Tests (src/core/task/__tests__/delegation-concurrent.spec.ts, existing delegation tests, and focused e2e coverage):

• maxConcurrency = 1: existing delegation behavior is preserved; parent is suspended/removed as before.
• maxConcurrency = 2: parent task's request loop remains in flight while child run() actually starts.
• Both parent and child appear in taskRegistry.getRunning() and do not share clineMessages.
• Parent persisted HistoryItem.status remains the intended delegated state while the live parent remains in memory.
• tryReserve() proves two concurrent reservations cannot both win the final permit.
• Fan-out with different parent/child modes and saved profiles: parent follow-up request continues using the parent's mode/profile/API config, while child request uses the child's requested mode/profile/API config.
• createTask() failure in fan-out releases the reserved permit.
• createTask() failure in non-fan-out restores the evicted parent.
• Metadata-persistence failure evicts the created child by id and rolls back or releases resources correctly.
• E2E subtask suite includes a fan-out smoke test and the full subtasks.test.ts remains green.

Acceptance Criteria

• maxConcurrency = 1 path is unchanged — all existing delegation tests pass.
• maxConcurrency = 2 fan-out path is only reachable via explicit config and an immediately reserved permit.
• Both parent and child are independently tracked in TaskRegistry with no shared message state.
• The running parent is isolated from child setup: no child mode/profile/API config leaks into parent task mode, live API handler, sticky profile, system prompt, or subsequent API request.
• All rollback paths release reserved permits and/or restore the parent before rethrowing.
• Focused unit/integration tests and the full subtask e2e file pass.

Explicitly Out of Scope

• showTaskWithId live-task focus switching and extension-side non-focused post suppression are Story [Story 3.2c] Extension-side task-scoping guard #370.
• Webview-side message rejection and per-task sequence handling are Story [Story 3.2d] Webview-side task-scoping guard + per-task seq #371.
• Child completion callback and live-parent result injection are Story [Story 3.3] Child completion callback and parent wakeup #372.