feat(deacon): implement bulletproof pause mechanism (gt-bpo2c) (#265)

Add multi-layer pause mechanism to prevent Deacon from causing damage:

Layer 1: File-based pause state
- Location: ~/.runtime/deacon/paused.json
- Stores: paused flag, reason, timestamp, paused_by

Layer 2: Commands
- `gt deacon pause [--reason="..."]` - pause with optional reason
- `gt deacon resume` - remove pause file
- `gt deacon status` - shows pause state prominently

Layer 3: Guards
- `gt prime` for deacon: shows PAUSED message, skips patrol context
- `gt deacon heartbeat`: fails when paused

Helper package:
- internal/deacon/pause.go with IsPaused/Pause/Resume functions

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

internal/deacon/pause.go

@@ -0,0 +1,88 @@
+// Package deacon provides the Deacon agent infrastructure.
+package deacon
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"time"
+)
+
+// PauseState represents the Deacon pause file contents.
+// When paused, the Deacon must not perform any patrol actions.
+type PauseState struct {
+	// Paused is true if the Deacon is currently paused.
+	Paused bool `json:"paused"`
+
+	// Reason explains why the Deacon was paused.
+	Reason string `json:"reason,omitempty"`
+
+	// PausedAt is when the Deacon was paused.
+	PausedAt time.Time `json:"paused_at"`
+
+	// PausedBy identifies who paused the Deacon (e.g., "human", "mayor").
+	PausedBy string `json:"paused_by,omitempty"`
+}
+
+// GetPauseFile returns the path to the Deacon pause file.
+func GetPauseFile(townRoot string) string {
+	return filepath.Join(townRoot, ".runtime", "deacon", "paused.json")
+}
+
+// IsPaused checks if the Deacon is currently paused.
+// Returns (isPaused, pauseState, error).
+// If the pause file doesn't exist, returns (false, nil, nil).
+func IsPaused(townRoot string) (bool, *PauseState, error) {
+	pauseFile := GetPauseFile(townRoot)
+
+	data, err := os.ReadFile(pauseFile) //nolint:gosec // G304: path is constructed from trusted townRoot
+	if err != nil {
+		if os.IsNotExist(err) {
+			return false, nil, nil
+		}
+		return false, nil, err
+	}
+
+	var state PauseState
+	if err := json.Unmarshal(data, &state); err != nil {
+		return false, nil, err
+	}
+
+	return state.Paused, &state, nil
+}
+
+// Pause pauses the Deacon by creating the pause file.
+func Pause(townRoot, reason, pausedBy string) error {
+	pauseFile := GetPauseFile(townRoot)
+
+	// Ensure parent directory exists
+	if err := os.MkdirAll(filepath.Dir(pauseFile), 0755); err != nil {
+		return err
+	}
+
+	state := PauseState{
+		Paused:   true,
+		Reason:   reason,
+		PausedAt: time.Now().UTC(),
+		PausedBy: pausedBy,
+	}
+
+	data, err := json.MarshalIndent(state, "", "  ")
+	if err != nil {
+		return err
+	}
+
+	return os.WriteFile(pauseFile, data, 0600)
+}
+
+// Resume resumes the Deacon by removing the pause file.
+func Resume(townRoot string) error {
+	pauseFile := GetPauseFile(townRoot)
+
+	err := os.Remove(pauseFile)
+	if err != nil && !os.IsNotExist(err) {
+		return err
+	}
+
+	return nil
+}