refactor: extract shared AgentStateManager pattern (gt-gaw8e)

- Create internal/agent package with shared State type and StateManager
- StateManager uses Go generics for type-safe load/save operations
- Update witness and refinery to use shared State type alias
- Replace loadState/saveState implementations with StateManager delegation
- Maintains backwards compatibility through re-exported constants

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/witness/types.go

@@ -3,20 +3,18 @@ package witness
 
 import (
 	"time"
+
+	"github.com/steveyegge/gastown/internal/agent"
 )
 
-// State represents the witness's running state.
-type State string
+// State is an alias for agent.State for backwards compatibility.
+type State = agent.State
 
+// State constants - re-exported from agent package for backwards compatibility.
 const (
-	// StateStopped means the witness is not running.
-	StateStopped State = "stopped"
-
-	// StateRunning means the witness is actively monitoring.
-	StateRunning State = "running"
-
-	// StatePaused means the witness is paused (not monitoring).
-	StatePaused State = "paused"
+	StateStopped = agent.StateStopped
+	StateRunning = agent.StateRunning
+	StatePaused  = agent.StatePaused
 )
 
 // Witness represents a rig's polecat monitoring agent.