Improve PR description template (temporalio#7706)

## What changed?
Removed useless sections. Improved others. Changed HTML comments to
_italic_ placeholders. Added GHA to validate placeholder removal (not
blocking for now).

## Why?
Team agreement.

## How did you test it?
- [ ] built
- [x] run locally and tested manually
- [ ] covered by existing tests
- [ ] added new unit test(s)
- [ ] added new functional test(s)

Author: alexshtin

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,18 +1,15 @@
 ## What changed?
-<!-- Describe what has changed in this PR -->
+_Describe what has changed in this PR._
 
 ## Why?
-<!-- Tell your future self why have you made these changes -->
+_Tell your future self why have you made these changes._
 
 ## How did you test it?
-<!-- How have you verified this change? Tested locally? Added a unit test? Checked in staging env? -->
+- [ ] built
+- [ ] run locally and tested manually
+- [ ] covered by existing tests
+- [ ] added new unit test(s)
+- [ ] added new functional test(s)
 
 ## Potential risks
-<!-- Assuming the worst case, what can be broken when deploying this change to production? -->
-
-## Documentation
-<!-- Have you made sure this change doesn't falsify anything currently stated in `docs/`? If significant
-new behavior is added, have you described that in `docs/`? -->
-
-## Is hotfix candidate?
-<!-- Is this PR a hotfix candidate or does it require a notification to be sent to the broader community? (Yes/No) -->
+_Any change is risky. Identify all risks you are aware of. If none, remove this section._

.github/workflows/check-pr-placeholders.yml

@@ -0,0 +1,59 @@
+name: Validate PR description for placeholder lines or empty sections
+
+on:
+  pull_request:
+    types: [opened, edited, synchronize, reopened]
+
+permissions:
+  pull-requests: read
+
+jobs:
+  validate-pr-description:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Validate PR description for placeholder lines or empty sections
+        uses: actions/github-script@v7
+        with:
+          script: |
+            const pr = await github.rest.pulls.get({
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              pull_number: context.payload.pull_request.number
+            });
+
+            const body = pr.data.body || '';
+            const lines = body.split(/\r?\n/);
+
+            let violations = [];
+
+            // Detect placeholder lines: entire line starts and ends with _
+            lines.forEach((line, idx) => {
+              if (/^_.*_$/.test(line.trim())) {
+                violations.push(`Line ${idx + 1}: Placeholder "${line.trim()}"`);
+              }
+            });
+
+            // Detect empty sections: look for headers like '## Why?' followed by no meaningful content
+            const requiredSections = ['## What changed?', '## Why?', '## How did you test it?'];
+            requiredSections.forEach((header) => {
+              const idx = lines.findIndex(line => line.trim().toLowerCase() === header.toLowerCase());
+              if (idx !== -1) {
+                let contentIdx = idx + 1;
+                while (contentIdx < lines.length && lines[contentIdx].trim() === '') {
+                  contentIdx++;
+                }
+                const nextLine = lines[contentIdx]?.trim();
+                if (!nextLine || /^## /.test(nextLine)) {
+                  violations.push(`Section "${header}" appears to be empty.`);
+                }
+              }
+            });
+
+            if (violations.length > 0) {
+              console.log("❌ PR description issues found:");
+              violations.forEach(v => console.log(`- ${v}`));
+              core.setFailed(`PR description must not contain placeholders or empty sections.`);
+            } else {
+              console.log("✅ PR description passed all checks.");
+            }