[Issacboy123]

Summary

This PR lets a running child alert its parent about important information before the child finishes. A child can report a blocker, risk, contradiction, or decision that may change the parent's plan. Reports reach only the correct parent, stay within fixed size limits, and can be saved for the next parent turn without starting a new provider request while the parent is idle.

This PR is stacked on feat/delete-owned-subagent-sessions, the completed persistent-subagent stack.

Reasoning

Controller-issued capabilities bind delivery to the exact current child record and live session, preventing a stale or same-name replacement child from selecting or reaching a recipient. A private child-tool injection lane keeps the tool unavailable to root and standalone SDK sessions while reserving its name from public custom tools. Reusing the root's existing FIFO steering and JSONL custom-message paths preserves established ordering and persistence behavior; active queued reports remain non-durable until delivery, and hidden delivery does not imply secrecy.

Changes

• Added the strict child-only escalate_to_parent tool with a 1,000-character message limit and acknowledgement that does not echo submitted content.
• Reserved the tool name from public custom tools and injected it only into controller-created fresh, restored-wake, and replacement-wake child sessions.
• Bound escalation callbacks to the exact current controller record and live running session, with rejection across reset, replacement, shutdown, and stale capability use.
• Added hidden FIFO active delivery with acceptance-ordered coalescing, one continuation per envelope, and sealing when the queued message enters parent context.
• Added non-triggering idle delivery through synchronous selected-branch append with in-memory rollback and isolated post-admission listener failures.
• Enforced an 8,000-character aggregate pending limit across active and idle reports, including release and branch-filtering behavior.
• Added root guidance that treats escalation content as untrusted delegated-agent data and focused authorization, delivery, persistence, rollback, and backpressure tests.

Testing

• node ../../node_modules/vitest/dist/cli.js --run test/suite/subagent-parent-messaging.test.ts test/suite/subagent-services.test.ts test/suite/subagent-registration.test.ts test/suite/subagent-actions.test.ts test/suite/subagent-coordination.test.ts test/suite/subagent-persistence.test.ts test/suite/agent-session-queue.test.ts test/subagent-controller.test.ts
• npm.cmd run check
• git diff --check