fix(policy): read policy.json only from the caller's own working tree

loadPolicy() read .llm-security/policy.json from whatever root it was
given, and every scanner passes the SCANNED TARGET: scan-orchestrator
(policyRoot = resolve(args.target)), entropy-scanner (thresholds and
suppression patterns), signature-scanner (sig.custom_rules_path and
enabled_families), trigger-scanner (phrase lists) and ast-taint-scanner
(enabled, python_path). A foreign/cloned target could raise its own
entropy thresholds, disable SIG families, supply its own SIG ruleset or
name the interpreter the AST scanner spawns — configuring the scan of
itself. Same defect class as S3b's .llm-security-ignore fix.

Chosen: move isOwnWorkingTree() to scanners/lib/own-working-tree.mjs (one
copy, reused by the orchestrator's ignore-file check) and make
loadPolicy() refuse an EXPLICIT root that is not the caller's own tree —
defaults plus one stderr line, same form as S3b — because one rule in one
function covers every scanner and a future call site cannot forget it.
The IMPLICIT root (CLAUDE_PROJECT_ROOT/cwd, what every hook uses) is the
caller's own project by construction and is read as before.
entropy-scanner's calibration.policy_source no longer reports an ignored
file as its source.

New tests/scanners/policy-scope.test.mjs was red on 0d37f5a (foreign
target: entropy finding silenced, custom SIG rule loaded, findings differ
from the same tree without policy.json, no stderr line) and is green now;
its own-tree scenario (known-positive) is green before and after. The 15
existing policy tests that placed own-tree fixtures under os.tmpdir() now
use tests/helpers/own-tree.mjs (fixture under $HOME, cwd set to it).

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

scanners/lib/own-working-tree.mjs

@@ -0,0 +1,38 @@
+// own-working-tree.mjs — Is a scan target the caller's own working tree?
+// Zero external dependencies.
+//
+// Configuration that lives INSIDE a scanned target (.llm-security-ignore,
+// .llm-security/policy.json and the custom SIG ruleset it can point at) is
+// honored only when the target is the caller's own working directory (or a
+// subdirectory of it), and NEVER when the target resolves under the OS temp
+// directory (where git-clone.mjs materializes clones) — the second check is
+// defense-in-depth for the case a caller's own cwd sits under tmpdir.
+// Otherwise a foreign/cloned target could configure the scan of itself.
+//
+// S3b (v8.1.0, 2026-09-22) introduced this check in scan-orchestrator.mjs for
+// the ignore file; S3c moved it here so policy-loader.mjs shares the one rule.
+
+import { resolve, sep } from 'node:path';
+import { realpathSync } from 'node:fs';
+import { tmpdir } from 'node:os';
+
+/**
+ * @param {string} targetPath
+ * @returns {boolean}
+ */
+export function isOwnWorkingTree(targetPath) {
+  let resolvedTarget;
+  let resolvedCwd;
+  let resolvedTmp;
+  try {
+    resolvedTarget = realpathSync(resolve(targetPath));
+    resolvedCwd = realpathSync(process.cwd());
+    resolvedTmp = realpathSync(tmpdir());
+  } catch {
+    return false;
+  }
+  if (resolvedTarget === resolvedTmp || resolvedTarget.startsWith(resolvedTmp + sep)) {
+    return false;
+  }
+  return resolvedTarget === resolvedCwd || resolvedTarget.startsWith(resolvedCwd + sep);
+}