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>

scanners/agent-listing-scanner.mjs

@@ -0,0 +1,71 @@
+/**
+ * AGT Scanner — Agent-listing always-loaded token budget
+ *
+ * Claude Code injects a listing of every active agent's name+description into the
+ * system prompt so it knows which subagents it can delegate to. That listing is
+ * re-sent on EVERY turn, whether or not a delegation happens — so with many
+ * installed agents it is a large always-loaded cost (on a heavily-plugged machine
+ * the dominant single always-loaded source).
+ *
+ * Detection:
+ *   CA-AGT-NNN  aggregate agent-listing estimate exceeds the listing budget (low)
+ *
+ * INTELLECTUAL-HONESTY CONTRACT (the reason this is `low`, not a hard finding):
+ * unlike the skill listing, the agent-listing mechanism is NOT documented (agents
+ * are absent from Claude Code's published context breakdown). So the finding is an
+ * INFERRED, UPPER-BOUND ESTIMATE, the budget is a config-audit heuristic (no
+ * documented agent allotment) anchored on a conservative 200k window, and the
+ * evidence discloses all of that. The cap, budget, and enumerate-and-measure step
+ * live in `lib/agent-listing-budget.mjs` — AGT only constructs the finding.
+ *
+ * Zero external dependencies.
+ */
+
+import { finding, scannerResult } from './lib/output.mjs';
+import { SEVERITY } from './lib/severity.mjs';
+import {
+  AGGREGATE_BUDGET_TOKENS,
+  BUDGET_CALIBRATION_NOTE,
+  measureActiveAgentListing,
+} from './lib/agent-listing-budget.mjs';
+
+const SCANNER = 'AGT';
+
+/**
+ * Main scanner entry point.
+ *
+ * @param {string} _targetPath unused (agent listing is HOME-scoped)
+ * @param {object} _discovery unused (ignores project discovery)
+ */
+export async function scan(_targetPath, _discovery) {
+  const start = Date.now();
+  const findings = [];
+
+  const { aggregate } = await measureActiveAgentListing();
+
+  if (aggregate.overBudget) {
+    findings.push(finding({
+      scanner: SCANNER,
+      severity: SEVERITY.low,
+      title: 'Aggregate agent listing may exceed the always-loaded budget',
+      description:
+        `The ${aggregate.scanned} active agents carry about ${aggregate.aggregateTokens} tokens of ` +
+        'name+description text that Claude Code injects every turn so it knows which subagents it can ' +
+        `delegate to — above the ${AGGREGATE_BUDGET_TOKENS}-token budget this scanner anchors on a 200k ` +
+        'context window. Every one of those tokens is re-sent on every turn whether or not you delegate. ' +
+        'Note: unlike the skill listing, the agent-listing always-loaded mechanism is inferred (not ' +
+        'documented), so this is an upper-bound estimate (see evidence).',
+      evidence:
+        `active_agents_scanned=${aggregate.scanned}; description_chars=${aggregate.aggregateChars}; ` +
+        `description_tokens~${aggregate.aggregateTokens}; budget@200k=${AGGREGATE_BUDGET_TOKENS} tok; ` +
+        `over_by~${aggregate.overBy} tok - ${BUDGET_CALIBRATION_NOTE}`,
+      recommendation:
+        'Shrink the always-loaded agent listing: disable plugins whose agents you do not use (the whole ' +
+        'agent block leaves the listing), remove dead user agents from ~/.claude/agents/, and trim long ' +
+        'agent descriptions toward their trigger phrases.',
+      category: 'token-efficiency',
+    }));
+  }
+
+  return scannerResult(SCANNER, 'ok', findings, aggregate.scanned, Date.now() - start);
+}