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

1 files changed, 16 insertions, 0 deletions

diff --git a/AGENTS.md b/AGENTS.md
index 5d4497a..5f28f2f 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -97,6 +97,22 @@ task deps <id>
 
 Shows the dependency tree for a task.
 
+### View Task Tree
+```bash
+task tree [<id>]
+```
+
+Shows task hierarchy with visual status indicators:
+- `[ ]` - Open
+- `[~]` - In Progress  
+- `[✓]` - Done
+
+Examples:
+```bash
+task tree                 # Show all epics with their children
+task tree t-abc123        # Show specific epic/task with its children
+```
+
 ### Export Tasks
 ```bash
 task export [--flush]