feat(infra): design-reviewer agent + PR helper (US-24 + US-25)

US-24 — Process hygiene UI:
- New .claude/agents/design-reviewer.md (model: opus, read-only) — visual + design-system reviewer that runs after frontend-builder and before code-reviewer. Audits alignment, DESIGN.md tokens, light/dark consistency, typo hierarchy, whitespace rhythm, responsive sanity at 1280x720, button convention, V1 a11y. Output format mirrors code-reviewer.
- Updated .claude/agents/frontend-builder.md DoD: screenshots are MANDATORY (one per feature/state introduced or modified, light+dark when theming is in scope). Hard block on "Dev server not started" — must be flagged explicitly. Screenshots feed the design-reviewer step.

US-25 — PR helper:
- scripts/open-pr.sh wraps `POST /api/v1/repos/{owner}/{repo}/pulls`. Detects host/owner/repo from `git remote get-url origin`, reads basic-auth credentials from `~/.git-credentials` (same source as `git push`, no token in env), uses jq to compose the multiline-safe payload. Validates args, prints PR URL on success, exits non-zero with the server message on failure.
- Makefile target `open-pr TITLE="..." BODY=path/to/body.md [BASE=main]` wraps the script with the same arg validation.
- README.md "Make targets" table extended.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

@@ -110,6 +110,7 @@ mimic/
 | `make test-frontend` | `npm run test -- --run` in `frontend/` |
 | `make test-e2e` | Playwright acceptance suite (container must be running) |
 | `make clean` | Remove container + volume + Python/Node caches |
+| `make open-pr TITLE="…" BODY=path` | Open a PR on the Gitea repo for the current branch via the REST API. Reads credentials from `~/.git-credentials` (same source as `git push`) — no token in env. Wraps `scripts/open-pr.sh`. Defaults `BASE=main`. |
 
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