Add KIMI_MAX_ACTIVE_ISSUES = 3 constant and a _count_active_kimi_issues()
helper that queries the Gitea API for open kimi-ready issues. Before
creating a new delegation issue, create_kimi_research_issue() now checks
the active count and returns a descriptive error if the cap is reached.
Also adds 13 new unit tests covering the cap constant, the count helper,
and cap enforcement (at/above/below the threshold).
Fixes#1304