* feat(daemon): add SSE heartbeat keepalive during long tool executions

Prevents idle-timeout disconnects from reverse proxies and sandbox shell
APIs (e.g. Sandock) when tools like image generation run for minutes
without producing stdout output.

- Emit ': heartbeat' SSE comment every 15s in both sandagentRun and
  codingRunStream handlers
- Expose setHeartbeatIntervalMs() for test overrides
- Add unit tests for heartbeat during slow execution and absence during
  fast responses

* chore: version bumps and changelogs

* feat(sandbox-sandock): increase default shell timeout from 5min to 30min

Agent coding sessions with tool executions (image generation, builds, etc.)
can easily exceed 5 minutes. 30 minutes is a safer default for long-running
tasks. Callers can still override via the timeout option.