Write the pull request description for the current branch.

Gather facts first; do not write from memory.
- Base branch: {{#if base_branch}}{{base_branch}}{{else}}main (confirm with `git merge-base`){{/if}}. Run `git log --oneline <base>..HEAD` and `git diff <base>...HEAD --stat`, then read the key hunks.
{{#if ticket}}- This work is tracked as {{ticket}}; link it.
{{/if}}{{#if notes}}- Author's notes (treat as intent, verify against the diff): {{notes}}
{{/if}}

Output Markdown with exactly these parts:
**Title**: under 70 characters, imperative mood, names the behavior change and not the files.
## Summary
Two or three sentences: what changes for a user or caller.
## Why
The problem or goal, one short paragraph. If the diff does not reveal it, write "Why: <author to fill>" instead of guessing.
## What changed
Bullets grouped by area, behavior first, files second.
## How to test
The exact commands and the expected result; mention any new test. If nothing automated covers the change, give the manual steps.
## Risk and rollback
What could break, who is affected, and how to revert (feature flag, revert commit, migration down).

Rules: describe what the diff does, not what you would have done. No filler ("This PR..."). Mark anything you inferred as (inferred). Do not claim tests ran unless you ran them.