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>

internal/rules/evaluate.go

@@ -0,0 +1,30 @@
+package rules
+
+// Finding ist das Ergebnis einer einzelnen zutreffenden Regel.
+type Finding struct {
+	RuleID      string
+	RuleVersion int
+	Severity    Severity
+	Title       string
+	Fix         string
+	Sources     []string
+}
+
+// Evaluate prüft alle Regeln gegen f und liefert ein Finding für jede
+// zutreffende Regel. Reihenfolge folgt der Reihenfolge von rules.
+func Evaluate(rules []Rule, f Facts) []Finding {
+	var findings []Finding
+	for _, r := range rules {
+		if r.Condition.Matches(f) {
+			findings = append(findings, Finding{
+				RuleID:      r.ID,
+				RuleVersion: r.Version,
+				Severity:    r.Severity,
+				Title:       r.Title,
+				Fix:         r.Fix,
+				Sources:     r.Sources,
+			})
+		}
+	}
+	return findings
+}