Move health monitor to standalone repository

[xbattlax]

Summary

Follow-up to the post-merge feedback on #36:

• move the maintained ROS 2 package, tests, simulator, and detailed docs to xbattlax/health-monitor
• publish the stable v0.1.0 release for vendoring
• replace the duplicated contribution files with a README linking to the maintained package
• keep the public OOMWOO interface contract in docs/SOFTWARE_INTERFACES.md
• document the shared ROS clock requirement for component timestamps
• document reliable/transient-local roster and stack-state QoS
• keep component and MCU heartbeat topics reliable/volatile so a stale healthy MCU heartbeat cannot be replayed to a late subscriber

The standalone implementation also fixes the roster startup race reported in the review comment: the roster subscription now requests TRANSIENT_LOCAL, RELIABLE, depth 1. The repository move follows the maintainer request.

Standalone verification

• 12 unit/adapter tests pass
• deterministic simulator passes
• package builds with colcon in ROS 2 Jazzy
• contract docs are installed under share/oomwoo_health_monitor/docs
• GitHub Actions core and ros-jazzy jobs pass
• workflow passes actionlint

Scope

This PR only removes the now-externalized duplicate implementation and updates the OOMWOO contract/link. Runtime integration can pin v0.1.0 from oomwoo-install in a separate change.