feat(pinet): support worker-owned subtree brokers

[tmustier]

Summary

• supersedes/reuses the stronger draft work from feat(pinet): stage worker subtree supervision #764 for feat(pinet): allow workers to spawn and supervise sub-worker trees #761 rather than keeping a narrower duplicate implementation
• keeps the typed worker-subtree hierarchy fields and schema migration v18 (parentAgentId, rootAgentId, treeDepth, spawnedByAgentId, supervisionState, launchId, subtreeRole, laneId)
• lets a normal follower worker become a local subtree broker with /pinet subtree start while remaining visible to the central broker
• launches real tmux-backed subtree children with pinet action=spawn or /pinet subtree spawn, instead of stopping at env-printing or missing_broker_connected_worker_launcher
• points children at the worker-owned broker via PINET_SOCKET_PATH, so the central broker sees only the parent while the subtree DB owns the child roster/messages
• routes subtree read, send, agents scope=subtree, reload, exit, and /pinet subtree stop through the supervising worker/sub-broker
• documents the workflow and records manual E2E smoke evidence in .research/761-pinet-worker-subtree-e2e.md

Refs #761. Supersedes draft #764.

Verification

• pnpm --filter @gugu910/pi-slack-bridge lint
• pnpm --filter @gugu910/pi-slack-bridge typecheck
• pnpm --filter @gugu910/pi-slack-bridge test -- pinet-tools.test.ts pinet-commands.test.ts runtime-mode.test.ts
• pnpm --filter @gugu910/pi-slack-bridge build
• pnpm lint
• pnpm typecheck
• pnpm test
• Manual tmux E2E smoke: parent /pinet subtree start, real child spawn, child report, parent read/reply/control, central-only-parent check, subtree-child DB check, and cleanup.