feat(connectors): complete ATLAS-WP-0003 — discovery connectors (Phase 2)

T01 connector_base + docs/discovery-connectors.md (read-only/stateless,
candidate->PR->promote; `candidate` added to schema status enum; candidates/
gitignored, excluded from gate).
T02 connector_reposcoping (repo-scoping facts -> candidates; graceful degrade).
T03 connector_gitconfig (deterministic scan; real .env -> secret-ref, no values;
verified 4 real candidates from ~/state-hub).
T04 connector_featurecontrol (feature-flag surfaces linking to feature-control
keys, no eval logic; FR-12).
T05 registry_health (unowned + stale detection).
Make targets: connect-gitconfig/reposcoping/featurecontrol, registry-health.

WP-0003 finished (5/5).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -174,3 +174,8 @@ cython_debug/
 # PyPI configuration file
 .pypirc
 
+
+# Discovery connector candidates are ephemeral output for PR review
+# (ATLAS-WP-0003); promote into registry/surfaces/ to track. See
+# docs/discovery-connectors.md.
+registry/surfaces/candidates/