chore: add PR template and document title/body convention

Formalise the PR-flow conventions while we install the PR-flow itself.

.gitea/pull_request_template.md auto-populates the PR body in Gitea
with five sections: Summary / Motivation / Implementation notes /
Verification (with CI-gate checkboxes + ADR/diagram update flags) /
Related. Sections can be left blank when irrelevant; the template
guides without adding ceremony. Header HTML comment reminds the
contributor of the PR title format and links to the full convention.

docs/development.md §5 (Conventional commit cycle) gains a 'PR
conventions' subsection that:
- explains why the PR title format matters (squash-merge subject on
  main, validated by commitlint in the CI 'commits' job)
- separates feature-branch commit hygiene (exploratory OK) from PR
  title hygiene (must conform)
- documents the type vocabulary (feat/fix/docs/style/refactor/perf/
  test/build/ci/chore/revert)
- proposes an optional scope vocabulary (apps, libs, cross-cutting
  domains like decisions/docs/ci/deps)
- describes the body template

No new ADR. The PR title format is derived from ADR-0007 (Conventional
Commits at the commit-msg layer) plus ADR-0015 (squash-merge means PR
title becomes the commit subject on main). The body template is
tactical guidance, not architectural.

.gitea/pull_request_template.md

@@ -0,0 +1,30 @@
+<!--
+PR title format — becomes the squash-merge subject on main, validated by commitlint.
+
+  <type>(<scope>): <short description>
+
+Examples:
+  feat(portal-shell): add user-preferences panel skeleton
+  fix(portal-bff): correct env var bracket access
+  docs(decisions): add ADR-0018 for security baseline
+  chore(deps): bump @nx/* to 22.7.2
+
+Imperative mood, lowercase, no trailing period, target ≤ 70 chars.
+See docs/development.md §5 for the full convention (types, scopes).
+-->
+
+## Summary
+
+## Motivation
+
+## Implementation notes
+
+## Verification
+
+- [ ] `pnpm ci:check` green locally
+- [ ] `pnpm ci:audit` green (or pre-existing drift acknowledged)
+- [ ] Tested manually:
+- [ ] Architecture diagram updated (if `docs/architecture.md` was affected)
+- [ ] ADR amended or added (if a decision changed)
+
+## Related