feat(config): add TruthConfig for truth scoring engine (#30)

Add TruthConfig struct to config.rs with all truth engine parameters:
- enabled, scoring_interval_seconds, batch_size, rescore_after_seconds
- pln_base_confidence, ecan_decay_rate, ecan_spread_factor
- contradiction_threshold, verification_threshold, cross_ref_limit

All settings configurable via OPENBRAIN__TRUTH__* env vars with sensible defaults.
Update .env.example with full documentation of new variables.
Update Config::load() builder and Default impl.

Part of #29

.env.example

@@ -42,3 +42,27 @@ OPENBRAIN__AUTH__ENABLED=false
 
 # Logging
 RUST_LOG=info,openbrain_mcp=debug
+
+# Truth Engine (optional)
+# Enable the background truth scoring worker. When enabled, stored memories
+# are continuously evaluated for truthfulness using PLN deduction, ECAN
+# attention economy, and cross-referencing against related memories.
+OPENBRAIN__TRUTH__ENABLED=false
+# Seconds between scoring cycles (default: 300 = 5 minutes)
+OPENBRAIN__TRUTH__SCORING_INTERVAL_SECONDS=300
+# Number of memories to score per cycle (default: 50)
+OPENBRAIN__TRUTH__BATCH_SIZE=50
+# Seconds before a scored memory is re-evaluated (default: 86400 = 24 hours)
+OPENBRAIN__TRUTH__RESCORE_AFTER_SECONDS=86400
+# PLN base confidence for deduction chains (0.0–1.0, default: 0.85)
+OPENBRAIN__TRUTH__PLN_BASE_CONFIDENCE=0.85
+# ECAN STI decay rate per cycle (0.0–1.0, default: 0.95)
+OPENBRAIN__TRUTH__ECAN_DECAY_RATE=0.95
+# ECAN attention spread factor (default: 0.05)
+OPENBRAIN__TRUTH__ECAN_SPREAD_FACTOR=0.05
+# Similarity threshold for contradiction detection (0.0–1.0, default: 0.85)
+OPENBRAIN__TRUTH__CONTRADICTION_THRESHOLD=0.85
+# Truth value threshold for "verified" categorization (0.0–1.0, default: 0.8)
+OPENBRAIN__TRUTH__VERIFICATION_THRESHOLD=0.8
+# Max related memories to cross-reference per scoring (default: 10)
+OPENBRAIN__TRUTH__CROSS_REF_LIMIT=10