feat: implement issue activity tracking system (issue #113)

- Add comprehensive IssueActivityTracker service with ActivityType enum and IssueActivity dataclass
- Implement full CLI interface with log, show, list, summary, delete, and import-activities commands
- Support activity logging with automatic period detection and cost allocation integration
- Add activity retrieval by issue, by period, with filtering and pagination
- Include activity summaries with statistics and breakdowns across issues and time periods
- Support bulk operations for activity import from JSON/CSV formats
- Integrate with existing finance schema using cost_periods and issue_activity_log tables
- Add 28 comprehensive test cases covering all functionality with 100% pass rate
- Enable both table and JSON output formats for all CLI commands

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

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

markitect/cli.py

@@ -6378,6 +6378,10 @@ cli.add_command(wishlist_group)
 # Register issue management commands
 cli.add_command(issues_group)
 
+# Register issue activity tracking commands
+from markitect.issues.activity_commands import activity as activity_group
+cli.add_command(activity_group)
+
 
 # Query Paradigm Commands - Issue #62
 @click.group()