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/facts.go

@@ -0,0 +1,23 @@
+package rules
+
+// Consideration ist die Gegenleistung für einen Beitrag, wie sie die
+// Extraktion (Stufe 1) liefert.
+type Consideration string
+
+const (
+	ConsiderationPaid    Consideration = "bezahlt"
+	ConsiderationInKind  Consideration = "sachbezug"
+	ConsiderationNone    Consideration = "keine"
+	ConsiderationUnclear Consideration = "unklar"
+)
+
+// Facts sind die Fakten aus der Extraktion (Stufe 1), auf denen die
+// Regelauswertung (Stufe 2) urteilt. Die Extraktion liefert diese Werte,
+// sie bewertet sie nicht — das Urteil fällt ausschließlich das Regelwerk.
+type Facts struct {
+	Platform            string        `json:"platform"`
+	Consideration       Consideration `json:"consideration"`
+	DisclosurePresent   bool          `json:"disclosure_present"`
+	DisclosureWording   string        `json:"disclosure_wording"`
+	DisclosureBeforeCut bool          `json:"disclosure_before_cut"`
+}