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>

go.mod

@@ -3,12 +3,18 @@ module github.com/netcell-it/deklarix
 go 1.26.6
 
 require (
-	github.com/golang-migrate/migrate/v4 v4.19.1 // indirect
+	github.com/golang-migrate/migrate/v4 v4.19.1
+	github.com/jackc/pgx/v5 v5.10.0
+	gopkg.in/yaml.v3 v3.0.1
+)
+
+require (
 	github.com/jackc/pgerrcode v0.0.0-20220416144525-469b46aa5efa // indirect
 	github.com/jackc/pgpassfile v1.0.0 // indirect
 	github.com/jackc/pgservicefile v0.0.0-20240606120523-5a60cdf6a761 // indirect
-	github.com/jackc/pgx/v5 v5.10.0 // indirect
 	github.com/jackc/puddle/v2 v2.2.2 // indirect
+	github.com/kr/text v0.2.0 // indirect
+	github.com/rogpeppe/go-internal v1.16.0 // indirect
 	golang.org/x/sync v0.18.0 // indirect
 	golang.org/x/text v0.31.0 // indirect
 )