feat: add Build Architecture Guard PR template

- Create structured template for build-related changes
- Include L1/L2/L3 change classification
- Require BUILDING.md updates for sensitive file changes
- Add artifact SHA256 validation for L3 changes

pull_request_template.md

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+# Build Architecture Guard PR Template
+
+## Change Level
+
+- [ ] Level: **L1** / **L2** / **L3** (pick one)
+
+**Why:** …
+
+## Scope & Impact
+
+- [ ] Files & platforms touched: …
+- [ ] Risk triggers (env / script flow / packaging / SW+WASM /
+  Docker / signing): …
+- [ ] Mitigations/validation done: …
+
+## Commands Run (paste exact logs/snips)
+
+- [ ] Web: `npm run build:web` / `:prod`
+- [ ] Electron: `npm run build:electron:dev` / package step
+- [ ] Mobile: `npm run build:android:test` / iOS equivalent
+- [ ] Clean/auto-run impacted scripts
+
+## Artifacts
+
+- [ ] Names + **sha256** of artifacts/installers:
+
+Artifacts:
+
+```text
+<name-1>  <sha256-1>
+<name-2>  <sha256-2>
+```
+
+## Docs
+
+- [ ] **BUILDING.md** updated (sections): …
+- [ ] Troubleshooting updated (if applicable)
+
+## Rollback
+
+- [ ] Verified steps (1–3 cmds) to restore previous behavior
+
+## L3 only
+
+- [ ] ADR link:
+
+ADR: https://…