Add a [heartbeat] section to KernelConfig so users can tune the
inactivity timeout that determines when agents are marked unresponsive.
Reactive agents (hands) that sit idle between infrequent requests were
getting marked as crashed after the hardcoded 180s default, causing
the first request after idle to fail.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
The docs listed `duckduckgo` as the config value but the actual serde
deserialization produces `duck_duck_go` — serde's rename_all = "snake_case"
on the DuckDuckGo enum variant inserts underscores at each word boundary.
Updated all three occurrences in configuration.md to match the real value.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
