Fix broker-managed Pinet follower startup mode

[tmustier]

Summary

• detect broker-managed tmux follower launches and keep startup runtime off instead of honoring a persistent broker/single mode before /pinet follow
• preserve explicit follower/autoFollow behavior when the launch is intentionally configured to auto-follow
• hide recently disconnected/ghost agents from pinet agents by default, with include_ghosts: true for diagnostics

Closes #748.

Tests

• pnpm vitest run slack-bridge/runtime-mode.test.ts slack-bridge/pinet-tools.test.ts
• pnpm typecheck
• pnpm lint
• pnpm test
• pre-push hook re-ran pnpm lint && pnpm typecheck && pnpm test

Notes

• RCA confirmed: broker-managed tmux workers inherit persistent slack-bridge settings; when those settings request broker mode, session startup attempts transitionToRuntimeMode(..., "broker") before the broker sends /pinet follow, causing the duplicate-broker error even though follow subsequently succeeds.