feat(leaderboard): ghost performance leaderboard and A/B testing

config.example.toml

@@ -379,3 +379,16 @@ include = ["."]
 # Paths/patterns to exclude from the snapshot.
 # (default: target/, .git/, .worktrees/, *.db, *.log)
 exclude = ["target/", ".git/", ".worktrees/", "*.db", "*.log"]
+
+# Ghost performance leaderboard and A/B testing.
+# Run `sparks leaderboard show` to view rankings, or `sparks leaderboard compare ghost-a ghost-b`.
+[leaderboard]
+enabled = true
+# Ghost name to A/B test against the default ghost. Omit or leave empty to disable A/B routing.
+# ab_test_ghost = "experimental-coder"
+# Fraction of requests routed to the challenger ghost (0.0 = never, 1.0 = always).
+ab_test_fraction = 0.1
+# Minimum recorded samples before a promotion recommendation is surfaced.
+min_samples_for_recommendation = 50
+# Success-rate improvement (as a fraction) required to trigger a promotion recommendation.
+promotion_threshold = 0.10

src/config.rs

@@ -73,6 +73,8 @@ pub struct Config {
     pub sonarqube: SonarqubeConfig,
     #[serde(default)]
     pub snapshot: SnapshotConfig,
+    #[serde(default)]
+    pub leaderboard: LeaderboardConfig,
     #[serde(skip)]
     inline_secret_labels: Vec<String>,
 }
@@ -1505,6 +1507,43 @@ impl ManagerConfig {
     }
 }
 
+// ── Leaderboard config ────────────────────────────────────────────────
+
+#[derive(Debug, Deserialize, Serialize, Clone)]
+pub struct LeaderboardConfig {
+    /// Enable leaderboard tracking (default: true)
+    #[serde(default = "default_lb_enabled")]
+    pub enabled: bool,
+    /// Ghost name to A/B test against the default ghost (None = disabled)
+    pub ab_test_ghost: Option<String>,
+    /// Fraction of requests routed to the challenger ghost (0.0-1.0, default: 0.1)
+    #[serde(default = "default_ab_fraction")]
+    pub ab_test_fraction: f64,
+    /// Minimum samples before auto-promotion recommendation (default: 50)
+    #[serde(default = "default_lb_min_samples")]
+    pub min_samples_for_recommendation: u64,
+    /// Success rate improvement threshold for auto-promotion (default: 0.10 = 10%)
+    #[serde(default = "default_lb_threshold")]
+    pub promotion_threshold: f64,
+}
+
+fn default_lb_enabled() -> bool { true }
+fn default_ab_fraction() -> f64 { 0.1 }
+fn default_lb_min_samples() -> u64 { 50 }
+fn default_lb_threshold() -> f64 { 0.10 }
+
+impl Default for LeaderboardConfig {
+    fn default() -> Self {
+        Self {
+            enabled: default_lb_enabled(),
+            ab_test_ghost: None,
+            ab_test_fraction: default_ab_fraction(),
+            min_samples_for_recommendation: default_lb_min_samples(),
+            promotion_threshold: default_lb_threshold(),
+        }
+    }
+}
+
 impl Default for Config {
     fn default() -> Self {
         Self {
@@ -1538,6 +1577,7 @@ impl Default for Config {
             alerts: AlertsConfig::default(),
             sonarqube: SonarqubeConfig::default(),
             snapshot: SnapshotConfig::default(),
+            leaderboard: LeaderboardConfig::default(),
             inline_secret_labels: Vec::new(),
         }
     }