fix(staging_check): B6 reads registry from running staging instance env

B6 false-FAILed because it built the project registry from the
launcher process-env via a host-path hack (sys.path.insert +
importlib.reload), not from the running staging instance. Run from the
host, ORCH_PROJECTS_JSON is unset -> default ET+ORCH registry -> false
FAIL -> spurious deploy-staging -> development rollback.

Variant (v) per ADR-001: remove the host-path hack; canonically run the
suite INSIDE orchestrator-staging via docker exec so src.projects
resolves from /app (PYTHONPATH) with .env.staging. Verdict logic
extracted into pure _evaluate_b6(known) -> (passed, detail) +
_known_project_ids_from_registry() / _run_b6() with deterministic FAIL on
source unavailability. deployer.md and STAGING_CHECK.md updated to the
docker exec command. src/projects.py, .env* and checks A/B4/B5/C
untouched.

Refs: ORCH-048

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

scripts/staging_check.py

@@ -8,8 +8,14 @@ Checks:
   Block C — E2E   (create task in SANDBOX → trigger pipeline via /webhook/plane
                    → verify branch + job enqueued → CLEANUP in finally)
 
-Usage (inside the container or with correct env set):
-    python3 scripts/staging_check.py [--base-url http://localhost:8501] [--mode stub|full-real]
+Usage — CANONICAL: run INSIDE the orchestrator-staging container (ORCH-048, ADR-001)
+so B6 reads the registry from the running instance's own env (.env.staging):
+    docker exec orchestrator-staging \
+      python3 /repos/orchestrator/scripts/staging_check.py \
+      --base-url http://localhost:8501 [--mode stub|full-real]
+
+Running from the host leaves ORCH_PROJECTS_JSON unset → B6 falls back to the
+default (ET+ORCH) registry → false FAIL. See docs/operations/STAGING_CHECK.md.
 
 Exit code: 0 = all PASS, non-zero = at least one FAIL.
 
@@ -214,6 +220,59 @@ SANDBOX_PROJECT_ID = "8c5a3025-4f9d-4190-b79f-fa06276bb27e"
 PROD_ET_PROJECT_ID = "7a79f0a9-5278-49cd-9007-9a338f238f9c"
 PROD_ORCH_PROJECT_ID = "8da6aa25-a60e-44d6-a1e2-d8ae59aa7d6a"
 
+B6_LABEL = "B6 Registry: sandbox present, prod ET/ORCH absent"
+
+
+def _evaluate_b6(known: set[str]) -> tuple[bool, str]:
+    """Pure verdict logic for the B6 registry-isolation check (ORCH-048).
+
+    PASS ⟺ SANDBOX ∈ known ∧ PROD_ET ∉ known ∧ PROD_ORCH ∉ known (TR-2).
+    ``detail`` keeps the human-readable ``sandbox=…, prod-ET=…, prod-ORCH=…``
+    format (TR-3). Isolated from any I/O so both outcomes are unit-testable
+    without a live staging instance or docker (02-trz §9, ADR-001).
+    """
+    sandbox_present = SANDBOX_PROJECT_ID in known
+    et_absent = PROD_ET_PROJECT_ID not in known
+    orch_absent = PROD_ORCH_PROJECT_ID not in known
+    passed = sandbox_present and et_absent and orch_absent
+    detail = (
+        f"sandbox={'YES' if sandbox_present else 'NO'}, "
+        f"prod-ET={'NO(good)' if et_absent else 'YES(BAD!)'}, "
+        f"prod-ORCH={'NO(good)' if orch_absent else 'YES(BAD!)'}"
+    )
+    return passed, detail
+
+
+def _known_project_ids_from_registry() -> set[str]:
+    """Registry of the *running staging instance* — its own process-env (ORCH-048).
+
+    The suite is canonically run INSIDE ``orchestrator-staging`` via
+    ``docker exec`` (ADR-001), so ``src.projects`` resolves through the
+    container's ``PYTHONPATH=/app`` to ``/app/src/projects.py`` and reads
+    ``ORCH_PROJECTS_JSON`` from ``.env.staging``. This reflects exactly the
+    registry the live instance serves webhooks with — no host-path hack, no HTTP
+    bootstrap dependency.
+    """
+    from src.projects import known_plane_project_ids
+    return known_plane_project_ids()
+
+
+def _run_b6(results: Results) -> None:
+    """Run the B6 registry-isolation check and record its verdict.
+
+    Builds the known-id set from the running instance's registry and applies
+    ``_evaluate_b6``. Any failure to obtain the registry yields a deterministic
+    FAIL with a clear detail (TR-4) — never an unhandled exception and never a
+    false PASS.
+    """
+    try:
+        known = _known_project_ids_from_registry()
+    except Exception as e:
+        results.add(B6_LABEL, False, f"registry source unavailable: {e}")
+        return
+    passed, detail = _evaluate_b6(known)
+    results.add(B6_LABEL, passed, detail)
+
 
 def block_b(results: Results):
     print(f"\n{_BOLD}[Block B] ACCESS{_RESET}")
@@ -260,28 +319,11 @@ def block_b(results: Results):
     except Exception as e:
         results.add("B5 Gitea: orchestrator-sandbox accessible, push=true", False, str(e))
 
-    # B6 — Registry: sandbox in known IDs, prod ET/ORCH NOT in known IDs
-    try:
-        # Import from inside the container (script runs in /repos/orchestrator context)
-        sys.path.insert(0, "/repos/orchestrator")
-        # Force reload to pick up container env
-        import importlib
-        if "src.projects" in sys.modules:
-            importlib.reload(sys.modules["src.projects"])
-        from src.projects import known_plane_project_ids
-        known = known_plane_project_ids()
-        sandbox_present = SANDBOX_PROJECT_ID in known
-        et_absent = PROD_ET_PROJECT_ID not in known
-        orch_absent = PROD_ORCH_PROJECT_ID not in known
-        ok = sandbox_present and et_absent and orch_absent
-        detail = (
-            f"sandbox={'YES' if sandbox_present else 'NO'}, "
-            f"prod-ET={'NO(good)' if et_absent else 'YES(BAD!)'}, "
-            f"prod-ORCH={'NO(good)' if orch_absent else 'YES(BAD!)'}"
-        )
-        results.add("B6 Registry: sandbox present, prod ET/ORCH absent", ok, detail)
-    except Exception as e:
-        results.add("B6 Registry: sandbox present, prod ET/ORCH absent", False, str(e))
+    # B6 — Registry: sandbox in known IDs, prod ET/ORCH NOT in known IDs (ORCH-048).
+    # Reads the registry of the running staging instance from its own process-env
+    # (canonical: docker exec inside orchestrator-staging — ADR-001). No host-path
+    # hack; deterministic FAIL if the registry source is unavailable (TR-4).
+    _run_b6(results)
 
 
 # ---------------------------------------------------------------------------