feat: add rules engine (internal/rules) with first two disclosure rules

internal/rules implements Stufe 2 from the core principle: the LLM
extracts facts, this deterministic engine judges them against versioned
YAML rules. Facts/Condition/Rule/Finding types, a loader that refuses to
load on a missing id/version or a duplicate rule id rather than silently
skipping a bad file, and Evaluate() matching facts against rules.

Two real rules grounded in verified research (see rules/OPEN.md for the
open questions that surfaced along the way):
- WK-001: no disclosure at all despite consideration (§ 5a Abs. 4 UWG,
  § 22 Abs. 1 MStV)
- WK-004: disclosure present but hidden behind a "mehr anzeigen" cut
  (§ 5a Abs. 4 UWG, Leitfaden der Medienanstalten, LG Köln 12.05.2026)

The two are deliberately disjoint (WK-004 requires disclosure_present=
true) so a post with no disclosure at all doesn't double-fire both
rules. Golden suite in testdata/golden/ covers both rules plus two clean
cases; it's this suite, not the UI, that's the actual asset per
CLAUDE.md.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

testdata/golden/organic-no-consideration.json

@@ -0,0 +1,11 @@
+{
+  "name": "Organischer Post ohne Gegenleistung, keine Kennzeichnung noetig",
+  "facts": {
+    "platform": "instagram",
+    "consideration": "keine",
+    "disclosure_present": false,
+    "disclosure_wording": "",
+    "disclosure_before_cut": false
+  },
+  "expected_findings": []
+}

testdata/golden/paid-disclosure-hidden-by-cut.json

@@ -0,0 +1,13 @@
+{
+  "name": "Bezahlter Post, Kennzeichnung vorhanden aber hinter 'mehr anzeigen' versteckt",
+  "facts": {
+    "platform": "instagram",
+    "consideration": "bezahlt",
+    "disclosure_present": true,
+    "disclosure_wording": "Werbung",
+    "disclosure_before_cut": false
+  },
+  "expected_findings": [
+    { "rule_id": "WK-004", "rule_version": 1, "severity": "hoch" }
+  ]
+}

testdata/golden/paid-disclosure-visible.json

@@ -0,0 +1,11 @@
+{
+  "name": "Bezahlter Post, Kennzeichnung korrekt vor der Kürzung sichtbar",
+  "facts": {
+    "platform": "tiktok",
+    "consideration": "sachbezug",
+    "disclosure_present": true,
+    "disclosure_wording": "Werbung",
+    "disclosure_before_cut": true
+  },
+  "expected_findings": []
+}

testdata/golden/paid-no-disclosure.json

@@ -0,0 +1,13 @@
+{
+  "name": "Bezahlter Post ohne jede Kennzeichnung",
+  "facts": {
+    "platform": "instagram",
+    "consideration": "bezahlt",
+    "disclosure_present": false,
+    "disclosure_wording": "",
+    "disclosure_before_cut": false
+  },
+  "expected_findings": [
+    { "rule_id": "WK-001", "rule_version": 1, "severity": "hoch" }
+  ]
+}