Replace single-page GET labels?limit=100 with api_get_all-backed
_repo_label_id_map so later-page labels (e.g. type:feature,
workflow-hardening) are not falsely rejected during full-set replacement.
Also add post-mutation verification, fix related MCP/CLI label inventory
call sites, and add multi-page regression tests for the #601 reconciler
failure mode.
Closes#627