Add --phase flag to filter tasks by phase

[axon-agent]

Summary

• Adds a --phase flag to axon get tasks that allows filtering tasks by their current phase
• Supports all task phases: Pending, Running, Waiting, Succeeded, Failed
• Multiple phases can be specified: --phase Running,Pending,Waiting
• Includes shell completion for phase values
• Includes validation with deterministic, clear error messages for invalid phase values
• Filtering works with all output formats (table, yaml, json) and with --all-namespaces

Usage examples

# Show only running tasks
axon get tasks --phase Running

# Show non-completed tasks
axon get tasks --phase Pending,Running,Waiting

# Combine with other flags
axon get tasks --phase Running -A -o yaml

Closes #422

Test plan

• Unit tests for validatePhases covering valid phases, invalid phases, empty input, and case sensitivity
• Unit tests for filterTasksByPhase covering single phase, multiple phases, all phases, and no matches
• Unit test verifying the --phase flag is properly registered on the get task command
• CI passes (make test, make verify, make build)

🤖 Generated with Claude Code

[cubic-dev-ai]

No issues found across 2 files

[gjkim42]

Lgtm
/squash-commits