feat: human approval gates for plan-approve-implement workflows

[nextlevelshit]

Context

Fabro has first-class human gate nodes (shape=hexagon) that pause execution and present choices via CLI or web UI. This enables plan-approve-implement patterns where humans review plans before code changes proceed. Rejected plans loop back to planning.

Wave currently has no human-in-the-loop mechanism. Pipelines run unattended. For Wave's use as a development tool (not just CI), human gates enable supervised workflows where the developer stays in control at critical decision points.

Design Goals — Best of Both Worlds

Combine Fabro's human gates with Wave's TUI/WebUI and persona system.

Gate Step Type

steps:
  - name: plan
    persona: navigator
    
  - name: approve-plan
    type: gate
    depends_on: [plan]
    prompt: "Review the implementation plan"
    choices:
      - label: "Approve"
        key: "a"
        target: implement
      - label: "Revise"
        key: "r"
        target: plan
      - label: "Abort"
        key: "q"
        target: _fail     # special: fail the pipeline
    timeout: 3600s         # 1 hour before auto-timeout
    default: approve       # on timeout, use this choice
    
  - name: implement
    persona: craftsman
    depends_on: [approve-plan]

Interaction Channels

1. CLI — prompt in terminal with keyboard shortcuts (like Fabro's [A] Approve / [R] Revise)
2. TUI — Bubble Tea modal with choice buttons
3. Web UI — browser notification + button panel
4. API — REST endpoint for programmatic approval (enables Slack/webhook integration)

Freeform Input

Gates can accept freeform text (revision notes, additional instructions):

  - name: approve-plan
    type: gate
    freeform: true          # allow text input alongside choices

The freeform text becomes available as an artifact for downstream steps.

Auto-Approve Mode

For CI/automated runs: wave run --auto-approve impl-issue -- "..." skips all gates, using default choices.

Context Population

After a gate decision, the following are set in run context:

• gate.<step_name>.choice — selected choice label
• gate.<step_name>.text — freeform text (if any)
• gate.<step_name>.timestamp — when decision was made

What Wave Keeps

• Persona system (gates don't need personas — they're human interaction points)
• Contract validation (can still validate before presenting to human)
• Workspace isolation (gate doesn't modify workspace)

What Wave Gains

• Supervised workflows — human stays in control at critical points
• Plan review — navigator creates plan, human approves, craftsman implements
• Quality gates — human sign-off before PR creation
• Revision loops — rejected work loops back with human feedback

Implementation Scope

1. Add type: gate step type to manifest schema
2. Gate executor in internal/pipeline/executor.go — pauses execution
3. CLI gate handler — terminal prompt with choices
4. TUI gate handler — Bubble Tea modal
5. Web UI gate handler — REST endpoint + notification
6. --auto-approve flag on wave run

Research Sources

• Fabro human gates: https://docs.fabro.sh/tutorials/plan-implement
• Fabro gate interaction: hexagon shape, edge labels with keyboard accelerators
• Fabro auto-approve: --auto-approve flag for CI