Implement task tree visualization command

Add 'task tree' command to show hierarchical task structure:

Usage:
  task tree # Show all epics with their children task tree <id> #
  Show specific epic/task with children

Features: - Visual status indicators: [ ] open, [~] in-progress,
[✓] done - Shows task type: [Epic] or [Task] - Indented display
for hierarchy - Shows namespace associations

Example output:
  t-PpXWsU [Epic] [ ] Task Manager Improvements [Omni/Task.hs]
    t-PpYZt2 [Task] [ ] Implement child ID generation t-PpZGVf [Task]
    [✓] Add filtering by type and parent

Updated AGENTS.md with usage examples.

Closes task t-PpZlbL

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