[demo] checkbox-approval UX

[logosc]

Demo only. This is a mock-up of what the plan-approval gate would
look like with a task checkbox instead of /symphony approve slash
commands. Click the checkbox below to see how GitHub renders it as a
clickable CTA. (This issue has no symphony:ready label, so the
orchestrator won't pick it up.)

Plan

The agent has analyzed the issue and proposes the following changes:

```

Scope

files_touched:

• HELLO_DEMO.md
  estimated_lines_added: 1
  estimated_lines_removed: 0
  risk_summary: trivial — adds one file at the repo root
  ```

Approval

• I approve this plan — writer-permission required.

How it works:

1. You toggle the checkbox above. GitHub edits this comment, flipping
   `- [ ]` to `- [x]` and emitting an `issue.edited` event.
2. `symphony-go`'s polling loop sees the edit on the next tick.
3. The orchestrator looks up the user who toggled it (via `reactions` or
   `event.actor`), verifies their permission level
   (`admin / maintain / write`), and on success transitions the label to
   `symphony:implementing`. The agent starts implementation.
4. If a user without write permission toggles it, the orchestrator
   un-toggles it (edits back to `[ ]`) and posts a one-line comment
   explaining why.

Compared to `/symphony approve`:

	slash command	checkbox
Click count	0 (must type)	1
Visibility	text in timeline	rendered widget
Audit	comment is searchable	edit history is searchable
Spoof-resistance	same — both gated by permission check	same

Comments asking questions about the plan are welcome and don't trigger
approval — only the explicit checkbox does.

Reject path

To reject, leave the checkbox unchecked and either:

• add the `symphony:stop` label (orchestrator will mark blocked, post
  a comment), or
• comment with your concerns; the orchestrator does nothing on free-text
  comments.