docs(github-pr-workflow): add Slack permalink formatting worked example

src/recipes/software-delivery/skills/github-pr-workflow/skill.md

@@ -118,6 +118,17 @@ Discussion: https://dembraneworkspace.slack.com/archives/<channel_id>/p<thread_t
 
 Drop the `.` from the `thread_ts` (e.g. `1778661386.209989` → `p1778661386209989`).
 
+### Worked Example: Mapping a Live Slack Thread Permalink
+
+When a PR is opened during a session, retrieve the channel ID and thread timestamp from your session context and format them exactly as below. Do not leave the dot (`.`) in the timestamp or the webhook notification route will fail (as happened historically on PR #182):
+
+- **Raw Slack Channel ID:** `C0884QPQF6W`
+- **Raw Slack Thread Timestamp:** `1785231017.536359`
+- **Formated Discussion Line:**
+  ```
+  Discussion: https://dembraneworkspace.slack.com/archives/C0884QPQF6W/p1785231017536359
+  ```
+
 **Why this is required in the PR body even when the Linear ticket also has the permalink:** the daemon's GitHub webhook receiver walks `PR body → Linear ticket → Slack thread` to find the right thread to post `ack()` notifications into when a review/CI/comment event lands. If the PR has no Linear ticket (small docs PRs, housekeeping fixes — happens often), the chain breaks unless the permalink is in the PR body too. Dual metadata closes that gap. See `src/recipes/software-delivery/skills/github-webhook-events/skill.md` for the chain logic.
 
 If there was no Slack thread (the PR was opened from a Linear ticket alone, or it's a Sam-noticed self-maintenance PR), write `Discussion: none (Sam-noticed during <session_id> on <date>)` so the chain knows there's no thread to ack into and Sam can still attribute the work.