[guidelines] side-quest-11-09-agent-session-phases.md: raw Unicode emoji in checkpoint heading instead of shortcode

## File reviewed
`workshop/side-quest-11-09-agent-session-phases.md`

## Violations

### GitHub emoji shortcodes for icons and visual elements
**Offending text:**
> ## ✅ Checkpoint

**Suggested fix:**
Replace the raw Unicode emoji with the required GitHub shortcode so the heading renders consistently: change `## ✅ Checkpoint` to `## :white_check_mark: Checkpoint`, matching the format used in all other workshop step checkpoints.

> Generated by [📋 Guidelines Enforcer](https://github.com/githubnext/gh-aw-workshop/actions/runs/34804091319) · copilot · auto · 76.1 AIC · ⌖ 5.15 AIC · ⊞ 9.5K · [◷](https://github.com/search?q=repo%3Agithubnext%2Fgh-aw-workshop+is%3Aissue+%22gh-aw-workflow-call-id%3A+githubnext%2Fgh-aw-workshop%2Fguidelines-enforcer%22&type=issues)
> - [x] expires on Sep 21, 2026, 4:00 AM UTC

Open workshop/side-quest-11-09-agent-session-phases.md and find the `## ✅ Checkpoint` heading. Replace the raw emoji with `:white_check_mark:` to match the other workshop checkpoint headings. Done means the heading reads `## :white_check_mark: Checkpoint` and no raw Unicode emoji remains there.

Written by the indexing model from the issue text.