feat: optimize and enhance IssueActivity class - Issue #126

Enhanced IssueActivity dataclass with convenient methods and properties:
- Added activity_type_value, activity_type_display properties
- Added formatted_date, formatted_datetime properties
- Added truncated_details property for display
- Added contains_keyword() and has_implementation_activity() methods
- Added to_dict() method for clean serialization

Simplified code across the codebase:
- Reduced JSON serialization from 18 lines to 1 line (94% reduction)
- Reduced implementation detection from 13 lines to 3 lines (77% reduction)
- Improved table formatting using property access
- Fixed test inconsistencies using proper IssueActivity objects
- Removed complex helper code for dict/dataclass handling

Benefits:
- Single source of truth for all IssueActivity operations
- Consistent interface across all usage patterns
- Better encapsulation and maintainability
- Enhanced code readability and reliability
- All tests passing (1329/1329)

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

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

markitect/issues/activity_commands.py

@@ -72,18 +72,7 @@ def show(issue_id: int, limit: int, offset: int, output_format: str):
 
         if output_format == 'json':
             import json
-            activity_data = []
-            for activity in activities:
-                data = {
-                    'id': activity.id,
-                    'issue_id': activity.issue_id,
-                    'activity_type': activity.activity_type.value,
-                    'activity_date': activity.activity_date.isoformat() if activity.activity_date else None,
-                    'period_id': activity.period_id,
-                    'activity_details': activity.activity_details,
-                    'created_at': activity.created_at.isoformat() if activity.created_at else None
-                }
-                activity_data.append(data)
+            activity_data = [activity.to_dict() for activity in activities]
             click.echo(json.dumps(activity_data, indent=2))
 
         else:
@@ -96,11 +85,11 @@ def show(issue_id: int, limit: int, offset: int, output_format: str):
             for activity in activities:
                 rows.append([
                     activity.id,
-                    activity.activity_type.value.title(),
-                    activity.activity_date.strftime('%Y-%m-%d') if activity.activity_date else 'N/A',
+                    activity.activity_type_display,
+                    activity.formatted_date,
                     activity.period_id or 'N/A',
-                    (activity.activity_details[:40] + '...') if activity.activity_details and len(activity.activity_details) > 40 else (activity.activity_details or ''),
-                    activity.created_at.strftime('%Y-%m-%d %H:%M') if activity.created_at else 'N/A'
+                    activity.truncated_details,
+                    activity.formatted_datetime
                 ])
 
             click.echo(tabulate(rows, headers=headers, tablefmt='grid'))
@@ -139,18 +128,7 @@ def list(period_id: Optional[int], activity_type: List[str], output_format: str)
 
         if output_format == 'json':
             import json
-            activity_data = []
-            for activity in activities:
-                data = {
-                    'id': activity.id,
-                    'issue_id': activity.issue_id,
-                    'activity_type': activity.activity_type.value,
-                    'activity_date': activity.activity_date.isoformat() if activity.activity_date else None,
-                    'period_id': activity.period_id,
-                    'activity_details': activity.activity_details,
-                    'created_at': activity.created_at.isoformat() if activity.created_at else None
-                }
-                activity_data.append(data)
+            activity_data = [activity.to_dict() for activity in activities]
             click.echo(json.dumps(activity_data, indent=2))
 
         else: