feat(acp): expose OBSERVER_PUBLISH_TICK as configuration

[fatima-n09]

Fixes #6830 

Summary

Expose the observer frame publish interval as
--observer-publish-tick-secs / BUZZ_ACP_OBSERVER_PUBLISH_TICK_SECS,
defaulting to 1 (preserving current behavior).

At 1 frame/s, telemetry consumes up to ~60/min of the agent's 120/min

message quota. Operators can now increase the tick to reclaim quota
headroom for real chat messages:

| tick | telemetry max/min | chat headroom (of 120) |
|------|-------------------|------------------------|
| 1s   | ~60               | ~60                    |
| 2s   | ~30               | ~90                    |
| 5s   | ~12               | ~108                   |

Changes

- config.rs: Add observer_publish_tick_secs to Args (clap) and
  ResolvedConfig, wired through from_cli() and test defaults.
- lib.rs: Replace hardcoded OBSERVER_PUBLISH_TICK constant with
  Duration::from_secs(observer_publish_tick_secs.max(1)) in the
  publish pacer. Remove the now-unused constant. Thread the value
  through spawn_relay_observer_publisher → run_relay_observer_publisher.

Validation

- cargo check -p buzz-acp — compiles clean (no new warnings)
- All existing observer tests pass (tick=1 preserves current behavior)
- min(1) guard prevents zero/negative tick values

Compatibility

Additive config surface. Default value preserves existing behavior.
No relay-side changes needed.