Support dynamic workflow phases - #19

E2E verification supplement

I also ran a local interactive Pi smoke test against this branch. The fixture and session files were temporary local files on my machine only; they are not part of this PR and are not needed to review the change.

Setup:

• Model: openai/gpt-5.5
• Thinking: low
• Loaded this extension explicitly from the checkout
• Disabled ambient skills, extensions, prompt templates, themes, and context files
• Disabled parent built-in tools so the parent agent had to use the workflow tool

Workflow shape used for the smoke test:

• meta had only name and description; no meta.phases
• Entered phase("Initial scan")
• Had a skipped Review branch
• Created Inspect API and Inspect UI phases from a loop
• Ran 3 subagents against a tiny local facts.md fixture

The interactive UI rendered the dynamic phase list without an empty skipped phase row:

Workflow completed
◆ Workflow: verify_local_workflow (3/3 done)
  ✓ Initial scan 1/1
    #1 ✓ initial scan
  ✓ Inspect API 1/1
    #2 ✓ inspect api
  ✓ Inspect UI 1/1
    #3 ✓ inspect ui

The saved workflow details had the expected single phase list:

{
  "phases": ["Initial scan", "Inspect API", "Inspect UI"],
  "agentCount": 3,
  "doneCount": 3,
  "errorCount": 0
}

Additional checks:

• The skipped Review branch did not render as Review 0/0
• No dynamicPhases field was present in the details object
• npm test passed with 17 tests