refactor runtime into modular services + split Slack routing

[LIU9293]

Summary

• Split packages/core/runtime.ts into focused runtime modules (session-bootstrap, open-request, request-runner, event-stream, pending-question, recovery, thread-queue, selection-reply, and helpers) while preserving behavior.
• Introduced an agent provider registry (packages/agents/registry.ts) and wired packages/agents/index.ts to select provider via ODE_AGENT_PROVIDER (default opencode).
• Refactored Slack integration by extracting inbound routing (packages/ims/slack/message-router.ts), throttled updates (packages/ims/slack/message-updates.ts), and thread history (packages/ims/slack/message-history.ts) from packages/ims/slack/client.ts.
• Updated final response handling so long outputs are posted below the status message instead of being cut by chat.update limits.

Plan Coverage

• ✅ Runtime decomposition: completed
• ✅ IM-specific button-selection isolation: completed
• ✅ Agent wiring modularization: completed (provider registry + guards)
• ✅ Slack client modular split: completed (router/transport helpers extracted)
• ✅ Testability improvements: completed for unit + contract baseline
• ✅ Final response length behavior: completed
• ⚠️ Remaining optional follow-up: add full fake-adapter runtime integration scenarios (stop/retry/question/recovery matrix) and mark live smoke tests separately in CI

Validation

• bun test packages/core/test
• bun test packages/agents/test

Notes

• Button selection is treated as optional IM capability; IMs without interactive selection can rely on plain message + question flow.