supervisor: gate wraps ANY cbm_mcp_handle_tool embedder — test binaries recurse into spawn chains #845

Discovered during the #828 distill (2026-07-04). The #827 index supervisor's gate in handle_index_repository wraps unconditionally (unless --index-worker/CBM_INDEX_SUPERVISOR=0), and the worker spawn resolves the CURRENT binary (self) — correct for the MCP server/CLI, wrong for any other embedder of cbm_mcp_handle_tool.

Observed: tests/repro's runner indexes via cbm_mcp_handle_tool → the supervisor spawns ' cli --index-worker index_repository ...' → the test binary ignores argv, selects suites via the inherited CBM_REPRO_ONLY env (or runs ALL suites when unset) → re-indexes → spawns grandchildren recursively. 11+ min idle process chains observed; the main test-runner escapes only by accident (argv-based suite selection). These recursive spawn trees during test runs also multiply kernel VM-map load (see the 2026-07-04 host panics).

Fix shape (product, not just tests): main() of the real binary marks itself as a supervisor-capable host (e.g. cbm_index_supervisor_mark_host()); the gate wraps ONLY when marked. Embedders/tests then run in-process by default. Belt+suspenders: export CBM_INDEX_SUPERVISOR=0 in the repro/test harness mains. Needs a RED reproduction first (tiny embedder that calls handle_tool and asserts no child spawn) per reproduce-first policy.