Add Jira-lite issue semantics for agent task boards

[Tweakzx]

Summary

Implements the first slice of #62: Jira-inspired, agent-native task semantics while preserving the existing lightweight board model.

Changes

• Adds task issue fields: issue_type, success_criteria, risk_level, reporter, and environment
• Adds a task_dependencies table with project-scoped dependency validation
• Adds agent-native statuses: waiting_for_approval and failed
• Extends store transitions, CLI task creation/board output, markdown/dashboard exports, and console task details
• Shows issue type, risk, success criteria, dependencies, approval/failure states, and related actions in the console
• Adds migration coverage and store tests for issue fields, dependencies, and new statuses

Validation

• PYTHONPATH=src python3 -m unittest discover -s tests

Closes #62