refactor(heartbeat): add configurable interval and channel-aware routing

feat(config): add heartbeat interval configuration with default 30 minutes

feat(state): migrate state file from workspace root to state directory

feat(channels): skip internal channels in outbound dispatcher

feat(agent): record last active channel for heartbeat context

refactor(subagent): use configurable default model instead of provider default

pkg/config/config.go

@@ -135,7 +135,8 @@ type SlackConfig struct {
 }
 
 type HeartbeatConfig struct {
-	Enabled bool `json:"enabled" env:"PICOCLAW_HEARTBEAT_ENABLED"`
+	Enabled  bool `json:"enabled" env:"PICOCLAW_HEARTBEAT_ENABLED"`
+	Interval int  `json:"interval" env:"PICOCLAW_HEARTBEAT_INTERVAL"` // minutes, min 5
 }
 
 type ProvidersConfig struct {
@@ -261,7 +262,8 @@ func DefaultConfig() *Config {
 			},
 		},
 		Heartbeat: HeartbeatConfig{
-			Enabled: true,
+			Enabled:  true,
+			Interval: 30, // default 30 minutes
 		},
 	}
 }