fix: address review feedback - schema completeness + timeout cleanup in finally

Reviewer: rwmjhb (CortexReach)

1. Schema completeness: add rerankTimeoutMs to openclaw.plugin.json retrieval schema and index.ts PluginConfig.retrieval
2. Timeout cleanup: move clearTimeout into finally block so timer is always cleared even on fast failure paths
3. Also update warning message to show actual configured timeout value

Closes #346

Author: jlin53882