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>

.claude/agents/frontend-builder.md

@@ -44,6 +44,21 @@ cd frontend && npm run test -- --run
 
 If any of these fail, fix the cause before reporting completion.
 
+### Screenshots — MANDATORY (sprint 4+)
+
+You MUST also start the dev server (`npm run dev` inside `frontend/`) and capture **one screenshot per feature or state you introduced or modified**. Concretely :
+
+- Every new page → 1 screenshot.
+- Every modified page → 1 screenshot of the new state.
+- Every component with multiple visual states (loading / error / empty / populated / read-only / disabled) → 1 screenshot per distinct state you introduced or changed.
+- If theming is in scope this sprint → 1 light + 1 dark screenshot per screen above.
+
+Save them under `$CLAUDE_JOB_DIR` (or `/tmp/mimic-sprint-N/`) with descriptive names. **List the absolute paths in your final summary, grouped per screen.**
+
+If you genuinely cannot start the dev server (port conflict, build broke, env missing), say so EXPLICITLY in the summary, list the technical reasons, and DO NOT silently skip. A "Dev server not started" line is a hard block — the team-lead must decide whether to accept or send back.
+
+Screenshots are the **design-reviewer**'s primary input. Without them, the design-review step cannot run, the sprint cannot ship.
+
 ## Output format (when you return to the team-lead)
 
 A short Markdown summary:
@@ -53,6 +68,7 @@ A short Markdown summary:
 - **Mismatches with API** (if any — flagged, not patched)
 - **Open questions / design ambiguities** (escalate, don't decide)
 - **Test results** (vitest summary, typecheck/lint status)
+- **Screenshots delivered** (absolute paths, grouped per screen, light + dark when in scope) — see § Before you finish
 - **CLAUDE.md rules that helped**
 
 ## Principles