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/rule.go

@@ -0,0 +1,54 @@
+package rules
+
+// Severity ist die Schwere eines Findings.
+type Severity string
+
+const (
+	SeverityLow    Severity = "niedrig"
+	SeverityMedium Severity = "mittel"
+	SeverityHigh   Severity = "hoch"
+)
+
+// Rule ist eine versionierte Regel aus einer YAML-Datei in rules/.
+// Regel-IDs werden nie umbenannt oder wiederverwendet — Änderungen an
+// einer Regel erhöhen die Version.
+type Rule struct {
+	ID        string    `yaml:"id"`
+	Version   int       `yaml:"version"`
+	Title     string    `yaml:"titel"`
+	Condition Condition `yaml:"bedingung"`
+	Severity  Severity  `yaml:"schwere"`
+	Sources   []string  `yaml:"fundstelle"`
+	Fix       string    `yaml:"korrektur"`
+}
+
+// Condition ist eine flache UND-Bedingung über Facts. Ein nil/leeres
+// Feld bedeutet "keine Einschränkung durch dieses Feld".
+type Condition struct {
+	Consideration       []Consideration `yaml:"gegenleistung,omitempty"`
+	DisclosurePresent   *bool           `yaml:"kennzeichnung_vorhanden,omitempty"`
+	DisclosureBeforeCut *bool           `yaml:"kennzeichnung_vor_kuerzung,omitempty"`
+}
+
+// Matches prüft, ob f alle gesetzten Bedingungsfelder erfüllt.
+func (c Condition) Matches(f Facts) bool {
+	if len(c.Consideration) > 0 {
+		found := false
+		for _, allowed := range c.Consideration {
+			if allowed == f.Consideration {
+				found = true
+				break
+			}
+		}
+		if !found {
+			return false
+		}
+	}
+	if c.DisclosurePresent != nil && *c.DisclosurePresent != f.DisclosurePresent {
+		return false
+	}
+	if c.DisclosureBeforeCut != nil && *c.DisclosureBeforeCut != f.DisclosureBeforeCut {
+		return false
+	}
+	return true
+}