docs: Add Activity comment sync to GitHub/GitLab

docs/collaboration/activity.md

@@ -13,41 +13,33 @@ Each check in your checklist has its own Activity panel. It records everything t
 
 Activity captures all events for a check:
 
-- **Created** - When the check is added to the checklist
-- **Approvals** - When the check is approved or unapproved
-- **Comments** - Questions, discussions, and clarifications about the check
-- **Description updates** - Changes to the check's description
+- **Created**: When the check is added to the checklist
+- **Approvals**: When the check is approved or unapproved
+- **Comments**: Questions, discussions, and clarifications about the check
+- **Description updates**: Changes to the check's description
 
 ![Activity panel in Recce Cloud](../assets/images/using-recce/checklist-review.png){: .shadow}
 
-## Using Activity
-
-### Discuss a Specific Check
+## When to Use
 
-Use Activity to have focused conversations about a validation:
+- **Requesting context**: Ask the developer about unexpected results or ask reviewers about the acceptable thresholds
+- **Documenting decisions**: Record the process of making a decision
+- **Tracking history**: See who approved, what questions were asked, and how descriptions changed
+- **Handoff scenarios**: Give the next reviewer context on past decisions
 
-- Ask why a particular diff result is expected
-- Request clarification on acceptable thresholds
-- Discuss edge cases the check might miss
-- Document why a check was approved despite warnings
+## Sync Comments to GitHub/GitLab
 
-### Track Check History
+Comments you post in Activity automatically sync to the PR or MR. Each comment appears as a new comment on GitHub or GitLab, with a link back to the specific check in Recce.
 
-Activity shows the lifecycle of each check:
+![Post a comment in Recce Activity](../assets/images/collaboration/recce-activity-mention.png){: .shadow}
 
-- Who approved it and when
-- What questions were asked before approval
-- How the description changed over time
-- Whether it was re-run after updates
+The comment appears on the PR/MR:
 
-This history helps new reviewers understand past decisions.
+![Comment posted to GitHub PR](../assets/images/collaboration/github-pr-comment-from-recce.png){: .shadow}
 
-## When to Use
+You can @mention teammates using their GitHub or GitLab username (e.g., `@john-doe`). They'll receive a notification through GitHub or GitLab. Use the exact username—Recce doesn't currently map display names to usernames.
 
-- **Requesting context** - Ask the developer about unexpected results
-- **Documenting decisions** - Explain why you approved despite a warning
-- **Iterating on checks** - Track changes as the developer updates code
-- **Handoff scenarios** - Give the next reviewer context on your findings
+This works the same way on GitLab.
 
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