fix(kernel): idle reactive agents triggering spurious background API

[Vn1k]

## Summary

Fixes a bug where agents with `[autonomous] max_iterations = N` in their
`agent.toml` were firing background API calls while idle — observed as
unprompted requests with 24K+ input tokens and `finish_reason: "length"`
up to 90 minutes after the last user message.

The default `assistant` agent ships with `[autonomous] max_iterations = 100`,
making this affect every default installation.

**Evidence — provider logs showing two calls from the same session:**

<img width="1110" height="104" alt="screenshot-2026-03-23_00-04-45" src="https://github.com/user-attachments/assets/7202938f-87f8-43f9-a825-44a7270912aa" />

The first row (03:42 PM, `finish_reason: length`, 24,582 tokens) fired
**93 minutes after the last user message** (02:09 PM, `finish_reason: stop`).
No user interaction occurred between the two calls.

## Changes

- **`heartbeat.rs`** — Only use `heartbeat_interval_secs` as the unresponsive
  timeout for agents with `Continuous` or `Periodic` schedule. Reactive agents
  now always use `default_timeout_secs` (180s) regardless of any `[autonomous]`
  config. Previously, the 30s default caused Reactive agents to be marked
  `Crashed` after just 60s of user inactivity.

- **`kernel.rs`** — Added two guards in the heartbeat monitor loop:
  - Unresponsive check: skip `Crashed` marking for Reactive agents — an idle
    conversational agent is not crashed, it is waiting for the next user message.
  - Recovery loop: if a Reactive agent ends up `Crashed`, reset it silently to
    `Running` without publishing a `HealthCheckFailed` event or entering the
    recovery counter, preventing the idle → Crashed → recovered → idle cycle
    that triggered background `send_message` calls with the full session context.
    
## Testing

- [x] `cargo clippy --workspace --all-targets -- -D warnings` passes
  (one pre-existing warning in `openfang-types/src/model_catalog.rs:311`
  unrelated to this fix — exists on `main` before these changes)
- [x] `cargo test --workspace` passes — 228/228 kernel tests pass
- [x] Live integration tested — no background API calls observed after 10+
  minutes of inactivity with `assistant` agent

## Security

- [x] No new unsafe code
- [x] No secrets or API keys in diff
- [x] User input validated at boundaries