feat(ecc2): add token/cost meter widget (#775)

- TokenMeter widget using ratatui Gauge with color gradient (green->yellow->red)
- Budget fields (cost_budget_usd, token_budget) in Config
- Aggregate cost display in status bar
- Warning state at 80%+ budget consumption
- Tests for gradient, config fallback, and meter rendering

ecc2/src/config/mod.rs

@@ -3,6 +3,7 @@ use serde::{Deserialize, Serialize};
 use std::path::PathBuf;
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
+#[serde(default)]
 pub struct Config {
     pub db_path: PathBuf,
     pub worktree_root: PathBuf,
@@ -11,6 +12,8 @@ pub struct Config {
     pub session_timeout_secs: u64,
     pub heartbeat_interval_secs: u64,
     pub default_agent: String,
+    pub cost_budget_usd: f64,
+    pub token_budget: u64,
     pub theme: Theme,
 }
 
@@ -31,6 +34,8 @@ impl Default for Config {
             session_timeout_secs: 3600,
             heartbeat_interval_secs: 30,
             default_agent: "claude".to_string(),
+            cost_budget_usd: 10.0,
+            token_budget: 500_000,
             theme: Theme::Dark,
         }
     }
@@ -52,3 +57,36 @@ impl Config {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::Config;
+
+    #[test]
+    fn default_includes_positive_budget_thresholds() {
+        let config = Config::default();
+
+        assert!(config.cost_budget_usd > 0.0);
+        assert!(config.token_budget > 0);
+    }
+
+    #[test]
+    fn missing_budget_fields_fall_back_to_defaults() {
+        let legacy_config = r#"
+db_path = "/tmp/ecc2.db"
+worktree_root = "/tmp/ecc-worktrees"
+max_parallel_sessions = 8
+max_parallel_worktrees = 6
+session_timeout_secs = 3600
+heartbeat_interval_secs = 30
+default_agent = "claude"
+theme = "Dark"
+"#;
+
+        let config: Config = toml::from_str(legacy_config).unwrap();
+        let defaults = Config::default();
+
+        assert_eq!(config.cost_budget_usd, defaults.cost_budget_usd);
+        assert_eq!(config.token_budget, defaults.token_budget);
+    }
+}