docs+ci: add PR packaging template + enforce it in CI

.github/PULL_REQUEST_TEMPLATE.md

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+## What I built
+- Staging models: `stg_trips.sql`, `stg_zones.sql` (+ their `_*.yml` tests/descriptions)
+- Mart: `fct_daily_borough_stats.sql` at grain one row per (pickup_borough, pickup_date)
+- Macro: `safe_divide.sql`
+- Singular test: `assert_avg_tip_pct_within_bounds.sql`
+- `dbt_utils` declared in `packages.yml`
+- Lineage screenshot: `docs/lineage.png`
+- Business answers: `reports/answers.md`
+- AI usage: `AI_ASSIST.md`
+
+## How to review
+- Models: read `models/marts/fct_daily_borough_stats.sql` (grain + measures) and the staging models.
+- Lineage: `docs/lineage.png`.
+- Answers: `reports/answers.md`.
+- AI usage: `AI_ASSIST.md`.
+
+## How to run
+From a clean clone, with your own Postgres access:
+
+```bash
+cp profiles.yml.example profiles.yml   # set schema to dev_<your_name>
+cp .env.example .env                   # fill PG_HOST, PG_USER, PG_PASSWORD
+source .env
+dbt deps
+dbt build                              # runs models + tests
+```
+
+`dbt debug` must end with `All checks passed!` first.
+
+> Data dependency: this builds into **your own** `dev_<name>` schema on the shared Postgres. A reviewer without that access relies on `docs/lineage.png` and `reports/answers.md` as the canonical evidence.
+
+## What reviewers should see (expected results)
+Fill in what your build actually produces:
+- `dbt build` result: <e.g. PASS=NN WARN=1 ERROR=0>
+- `fct_daily_borough_stats` row count: <e.g. ~180>
+- Grain: one row per (pickup_borough, pickup_date)
+- One business answer sample: <e.g. Manhattan has the most trips>
+
+## Known limitations / out of scope
+- <e.g. the singular test WARNs rather than errors by design; incremental not implemented>
+- Write "none" if everything in the assignment is done and working.
+
+## Self-check
+- [ ] `bash .hyf/test.sh` passes
+- [ ] `dbt build` completes with no ERROR
+- [ ] `fct_daily_borough_stats` exists as a **table** at the stated grain
+- [ ] No credentials committed (`profiles.yml` / `.env` are gitignored)
+- [ ] `docs/lineage.png` is committed

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

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+name: PR body check
+
+# Fails the check when a pull request description is missing the sections from
+# .github/PULL_REQUEST_TEMPLATE.md. GitHub only auto-fills that template in the
+# web "compose" form and in `gh pr create` with no --body; a PR opened through
+# the REST API or `gh pr create --body "..."` (the path most AI tools take)
+# silently skips it. This check is the only thing that actually enforces it.
+#
+# Recovery is automatic: editing the PR description fires the `edited` event and
+# re-runs this check with the new body. No new commit or manual re-run needed.
+
+on:
+  pull_request:
+    types: [opened, edited, reopened, synchronize]
+    branches: [main]
+
+permissions:
+  contents: read
+
+jobs:
+  check:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check required sections are present
+        env:
+          PR_BODY: ${{ github.event.pull_request.body }}
+        run: |
+          set -euo pipefail
+          required=(
+            "## What I built"
+            "## How to review"
+            "## How to run"
+            "## What reviewers should see"
+            "## Self-check"
+          )
+          missing=()
+          for section in "${required[@]}"; do
+            if ! printf '%s' "$PR_BODY" | grep -qiF "$section"; then
+              missing+=("$section")
+            fi
+          done
+          if [ ${#missing[@]} -ne 0 ]; then
+            echo "::error::Your PR description is missing required sections. Start from the template (.github/PULL_REQUEST_TEMPLATE.md) and keep these headings:"
+            for m in "${missing[@]}"; do echo "  - $m"; done
+            echo ""
+            echo "If you (or an AI tool) opened this PR without the template, click 'Edit' on the"
+            echo "PR description, paste the template, and fill it in. Editing the description"
+            echo "re-runs this check automatically. A complete PR is easy to review and"
+            echo "reproducible: that is part of the assignment."
+            exit 1
+          fi
+          echo "All required PR sections present."