feat(pinet): stage worker subtree supervision

[tmustier]

Summary

Refs #761.

Stages the safe broker-side groundwork for Pinet worker subtrees using real broker-connected followers, not local Agent subagents:

• adds typed agent hierarchy fields and schema migration v18 (parentAgentId, rootAgentId, treeDepth, spawnedByAgentId, supervisionState, launchId, subtreeRole, laneId)
• accepts hierarchy metadata during real follower registration and rejects dead parents, self-parenting, and hierarchy cycles
• scopes A2A routing so parent/child/ancestor/descendant supervision works while unrelated workers cannot message supervised children
• keeps broker emergency override trusted via broker self id rather than follower-supplied role: broker metadata
• excludes supervised/orphaned/stopping children from default broadcast/backlog assignment while preserving explicit preferred target recovery
• adds lifecycle notices for child exit and parent orphaning/reaping
• extends pinet action=agents with subtree inspection (scope=children|subtree, parent_agent)
• adds docs/design note in plans/761-pinet-worker-subtrees.md

#406 caveat / spawn status

pinet action=spawn is intentionally a safe validation surface for now. It validates the child task/scope request, but fails safely with missing_broker_connected_worker_launcher until the #406 broker-connected worker bootstrap/launcher exists.

This PR does not spawn local Agent subagents and does not claim end-to-end child follower startup yet.

Validation

• pnpm --filter @gugu910/pi-broker-core typecheck
• pnpm --filter @gugu910/pi-slack-bridge typecheck
• pnpm --filter @gugu910/pi-broker-core lint
• pnpm --filter @gugu910/pi-slack-bridge lint
• pnpm --filter @gugu910/pi-broker-core test — 3 files / 44 tests
• pnpm --filter @gugu910/pi-slack-bridge test -- broker/helpers.test.ts broker/integration.test.ts pinet-tools.test.ts pinet-mesh-ops.test.ts runtime-agent-context.test.ts — 76 files / 1344 tests
• pnpm --filter @gugu910/pi-broker-core build
• pnpm --filter @gugu910/pi-slack-bridge build
• Push pre-push hook also passed full repo pnpm lint && pnpm typecheck && pnpm test before branch upload.