Expose heartbeat default_timeout_secs in config.toml

[pbranchu]

Summary

• Add a [heartbeat] section to KernelConfig with a default_timeout_secs field (default: 180)
• Wire start_heartbeat_monitor to read the configured timeout instead of using the hardcoded default
• Add tests for deserialization and default values
• Document the new config section in docs/configuration.md

Problem

Reactive agents (hands) that sit idle between infrequent requests get marked as crashed after the hardcoded 180-second inactivity timeout. This causes the first request after an idle period to fail with a health-check error, requiring the kernel to recover the agent before it can serve the request.

Users with hands that are called infrequently (e.g., a code-review hand invoked a few times per day) need to increase this timeout without modifying source code.

Usage

[heartbeat]
default_timeout_secs = 600   # 10 minutes for infrequently-used hands

Per-agent heartbeat_interval_secs in autonomous config continues to override this global default.

Test plan

• cargo test -p openfang-types -- config::tests::test_heartbeat passes (4 new tests)
• cargo test -p openfang-kernel -- heartbeat::tests::test_heartbeat_config_custom_timeout passes
• Verify a kernel booted with [heartbeat] default_timeout_secs = 600 logs the custom value
• Verify omitting the [heartbeat] section preserves the 180s default

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