Agent sessions: coordinate independently scoped child sessions

Some tasks genuinely require independent units of work across repositories, workspaces, branches/worktrees, providers, or lifecycles. Today, users and agents must manually create those sessions, track progress, move context between them, and decide when their results are ready to integrate.

## Product model

A session is the user-visible unit of work: one workspace/worktree, lifecycle, and aggregate diff. Parallel subtasks that share that unit should use chats within the session.

Cross-session delegation is the complementary path for work that must be independently scoped and resumable.

## Example workflow

A user asks an agent to implement a feature spanning separate client and server repositories. The creator session should be able to:

1. Resolve each workspace and create one child session per independently scoped workstream.
2. Give each child a focused task, label, and optional model/provider.
3. Continue coordinating while the children run independently.
4. Be notified when a child finishes, needs input, or fails.
5. Inspect each result and summarize or integrate it for the user.

## Proposed experience

- Prefer same-session chats when parallel work should share one workspace and aggregate diff.
- Allow an agent to create and message child sessions for independent repositories, workspaces, branches/worktrees, providers, or lifecycles.
- Keep the creator session and its aggregate result primary.
- Surface child relationships, labels, status, workspace/worktree, and linked PR when useful, without making implementation-detail children dominate the session list.
- Let users open a child to inspect or redirect independently scoped work.
- Return completion or failure signals without interrupting an active creator turn.
- Let the creator collect bounded child context and synthesize the final result.

## Acceptance criteria

- Tool guidance distinguishes same-session chat delegation from independently scoped child sessions.
- A creator can run at least two independently scoped child sessions concurrently.
- Each child has an explicit task, workspace, status, and discoverable relationship to its creator.
- The creator receives reliable completion, input-needed, and error signals.
- The creator can inspect child results without manual copy/paste.
- Users can tell where child changes live and how to open or integrate them.
- Delegation honors existing confirmation, trust, permission, and concurrency controls.

## Related issues

- #282818 — one-click delegation to multiple agents
- #286547 — multiple background agents from Copilot Chat
- #296469 — dynamic cross-model delegation
- #317195 — notifications when sessions need attention or finish
- #330948 — workspace resolution for agent session tools
- #331541 — answer child input from the creator session
- #331554 — surface cross-session delegation without obscuring aggregate work

Implementation:

- #331518 — cross-session delegation relationships and notifications
- #331525 — workspace resolution through existing session tools