Restructure agent config: unified .agents/skills + wiki/architecture

Replace per-tool rule trees (.cursor/rules, .claude/rules, .codex/rules)
with a single wiki/architecture/ source — 11 pages + README — readable
as plain markdown by any agent. Canonical skills live in .agents/skills/;
.claude/skills, .cursor/skills, .codex/skills are directory symlinks.

AGENTS.md is the entrypoint (rewritten as a lean overview, no per-tool
path lists). CLAUDE.md, GEMINI.md, and .github/copilot-instructions.md
all point to it.

Add open-pr skill (uses .github/pull_request_template.md as the source
of truth for the PR body) and remove the dangling .claude/CLAUDE.md
relative symlink.

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

wiki/architecture/creating-rules.md

@@ -0,0 +1,42 @@
+# Creating Architecture Pages
+
+*How to add or update a page under `wiki/architecture/`.*
+
+The pages in `wiki/architecture/` are the canonical source of architectural rules. AI agents (Claude, Codex, Cursor, Gemini) read them on demand via `AGENTS.md`.
+
+## Workflow
+
+1. Pick a focused topic (one concept per page — layers, events, registry, …).
+2. Create `wiki/architecture/<slug>.md`. Slugs are short, kebab-case.
+3. Add an entry to `wiki/architecture/README.md` so the new page is discoverable.
+4. If the page should be read for PR reviews, link it from `.agents/skills/review-architecture/SKILL.md`.
+
+## Page format
+
+```markdown
+# Page Title
+
+*One-line italic description of what this page covers.*
+
+Applies to: `path/glob/**`.
+
+Short intro paragraph.
+
+## Section
+
+Concrete guidance with code examples and rules.
+```
+
+The italic description and `Applies to:` line replace the old Cursor frontmatter — they're plain markdown so every agent sees them.
+
+## Good practice
+
+- Keep a page focused on one concept. Split if it grows past ~500 lines.
+- Lead with the rule, follow with the example. Show the correct shape before listing prohibitions.
+- Reference real source files with a plain backtick path (e.g. `packages/core/src/schema/base.ts`).
+- Add a new page when the same mistake has been made twice — not preemptively.
+- Never duplicate content across pages. Link instead.
+
+## Existing pages
+
+See `wiki/architecture/README.md` for the current index.