feat(scanner): add AGT agent-listing always-loaded budget finding (v5.9 B1)

New AGT scanner flags the aggregate always-loaded cost of the agent listing
(every active agent's name+description is injected each turn so the model
knows what it can delegate to). Mirrors the SKL skill-listing pattern but
encodes the key honesty caveat: the agent-listing mechanism is INFERRED
(agents are absent from Claude Code's documented context breakdown), so the
token figure is an UPPER-BOUND estimate and the aggregate budget is a
config-audit heuristic anchored on 200k — not a CC-documented allotment.

- scanners/agent-listing-scanner.mjs + scanners/lib/agent-listing-budget.mjs
- 8 TDD tests (tests/scanners/agent-listing-scanner.test.mjs)
- AGT folds into the Token Efficiency health area (scoring.mjs)
- byte-stability: AGT added to strip-added-scanner (frozen v5.0.0 baselines
  left untouched, same as OST/OPT); SC-5 default-output snapshot refreshed
- suite 1168 -> 1176 green

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/snapshots/default-output/scan-orchestrator.json

@@ -569,6 +569,20 @@
           "info": 0
         }
       },
+      {
+        "scanner": "AGT",
+        "status": "ok",
+        "files_scanned": 0,
+        "duration_ms": 0,
+        "findings": [],
+        "counts": {
+          "critical": 0,
+          "high": 0,
+          "medium": 0,
+          "low": 0,
+          "info": 0
+        }
+      },
       {
         "scanner": "OST",
         "status": "ok",
@@ -610,7 +624,7 @@
       "risk_score": 11,
       "risk_band": "Medium",
       "verdict": "PASS",
-      "scanners_ok": 14,
+      "scanners_ok": 15,
       "scanners_error": 0,
       "scanners_skipped": 1
     }