chore: Update title guidance in pull requests template (DBTP-1862)

Author: WillGibson

.github/pull_request_template.md

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+Addresses https://uktrade.atlassian.net/browse/DBTP-<ticket>.
+
+---
+## Checklist:
+
+### Title:
+- [ ] Conforms to [our pull request title guidance](https://uktrade.atlassian.net/wiki/spaces/DBTP/pages/4402020487/Git+housekeeping#Pull-request-titles). E.g. `feat: Add new feature (DBTP-1234)` or `chore: Correct typo (off-ticket)`
+
+### Description:
+- [ ] Link to ticket included (unless it's a quick out of ticket thing)
+- [ ] Includes tests (or an explanation for why it doesn't)
+- [ ] Includes any applicable changes to the documentation in this code base

.github/workflows/pull-request-checks.yml

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+name: "Lint PR"
+
+on:
+  pull_request:
+    types:
+      - opened
+      - edited
+      - synchronize
+
+permissions:
+  pull-requests: read
+
+jobs:
+  main:
+    name: Validate PR title
+    runs-on: ubuntu-latest
+    steps:
+      - uses: amannn/action-semantic-pull-request@v5
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        with:
+          # Check the subject matches guidance from https://uktrade.atlassian.net/wiki/spaces/DBTP/pages/4402020487/Git+housekeeping?force_transition=f6ce49ea-c406-402b-ac37-ed6145b46259#Pull-request-titles
+          subjectPattern: ^[A-Z].* \([a-zA-Z]{1,}-[a-zA-Z0-9]{1,}\)+$
+          subjectPatternError: |
+            The subject "{subject}" found in the pull request title "{title}"
+            didn't match the expected pattern. E.g. "feat: Add new feature (DBTP-1234)".