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/agent/state.go

@@ -0,0 +1,76 @@
+// Package agent provides shared types and utilities for Gas Town agents
+// (witness, refinery, deacon, etc.).
+package agent
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+
+	"github.com/steveyegge/gastown/internal/util"
+)
+
+// State represents an agent's running state.
+type State string
+
+const (
+	// StateStopped means the agent is not running.
+	StateStopped State = "stopped"
+
+	// StateRunning means the agent is actively operating.
+	StateRunning State = "running"
+
+	// StatePaused means the agent is paused (not operating but not stopped).
+	StatePaused State = "paused"
+)
+
+// StateManager handles loading and saving agent state to disk.
+// It uses generics to work with any state type.
+type StateManager[T any] struct {
+	stateFilePath  string
+	defaultFactory func() *T
+}
+
+// NewStateManager creates a new StateManager for the given state file path.
+// The defaultFactory function is called when the state file doesn't exist
+// to create a new state with default values.
+func NewStateManager[T any](rigPath, stateFileName string, defaultFactory func() *T) *StateManager[T] {
+	return &StateManager[T]{
+		stateFilePath:  filepath.Join(rigPath, ".runtime", stateFileName),
+		defaultFactory: defaultFactory,
+	}
+}
+
+// StateFile returns the path to the state file.
+func (m *StateManager[T]) StateFile() string {
+	return m.stateFilePath
+}
+
+// Load loads agent state from disk.
+// If the file doesn't exist, returns a new state created by the default factory.
+func (m *StateManager[T]) Load() (*T, error) {
+	data, err := os.ReadFile(m.stateFilePath)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return m.defaultFactory(), nil
+		}
+		return nil, err
+	}
+
+	var state T
+	if err := json.Unmarshal(data, &state); err != nil {
+		return nil, err
+	}
+
+	return &state, nil
+}
+
+// Save persists agent state to disk using atomic write.
+func (m *StateManager[T]) Save(state *T) error {
+	dir := filepath.Dir(m.stateFilePath)
+	if err := os.MkdirAll(dir, 0755); err != nil {
+		return err
+	}
+
+	return util.AtomicWriteJSON(m.stateFilePath, state)
+}