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>

.agents/skills/open-pr/SKILL.md

@@ -0,0 +1,97 @@
+---
+name: open-pr
+description: Open a pull request on pascalorg/editor using the repo's PR template. Use when the user asks to open/create a PR, push and PR, or ship a branch in the editor repo.
+allowed-tools: Bash(git *) Bash(gh *) Read
+---
+
+Open a pull request against `pascalorg/editor` from the current branch.
+
+## 1. Pre-flight
+
+```bash
+git status                # confirm working tree state
+git branch --show-current # confirm we're on a feature branch, not main
+git log --oneline main..HEAD
+```
+
+Stop if:
+- The current branch is `main`. Ask the user to create a feature branch first.
+- The branch has no commits ahead of `main`. Nothing to open a PR for.
+- There are uncommitted changes the user hasn't asked to commit.
+
+Run a build sanity check if the change is non-trivial:
+
+```bash
+bun typecheck
+bun build
+```
+
+Don't open the PR with a broken build.
+
+## 2. Read the PR template
+
+The template is at `.github/pull_request_template.md`. Read it before composing the body — the section headings and checklist items are the source of truth, not your memory of them.
+
+```bash
+cat .github/pull_request_template.md
+```
+
+Mirror the template exactly:
+
+- `## What does this PR do?` — one paragraph or a short bullet list. Link related issues with `Fixes #123` when applicable.
+- `## How to test` — numbered, concrete reviewer steps (commands to run, what to click, expected outcome).
+- `## Screenshots / screen recording` — if the change is visual, paste a recording link or note that one will be added. If purely non-visual (refactor, internal API), say so explicitly so the reviewer knows nothing is missing.
+- `## Checklist` — copy the boxes verbatim, ticking the ones already verified.
+
+## 3. Push and open
+
+```bash
+git push -u origin HEAD
+```
+
+Check for an existing PR first:
+
+```bash
+gh pr view --json url 2>/dev/null
+```
+
+If none exists, create it. Pass the body via HEREDOC to preserve markdown formatting:
+
+```bash
+gh pr create --title "short, scope-prefixed title" --body "$(cat <<'EOF'
+## What does this PR do?
+
+<one-paragraph description; link issues>
+
+## How to test
+
+1. <step>
+2. <step>
+3. <step>
+
+## Screenshots / screen recording
+
+<link or "N/A — non-visual change">
+
+## Checklist
+
+- [x] I've tested this locally with `bun dev`
+- [x] My code follows the existing code style (run `bun check` to verify)
+- [ ] I've updated relevant documentation (if applicable)
+- [x] This PR targets the `main` branch
+EOF
+)"
+```
+
+Keep the title under ~70 characters. Use a scope prefix when there's an obvious one (`viewer:`, `core:`, `editor:`, `mcp:`).
+
+If a PR already exists, print its URL and stop — don't recreate.
+
+## 4. Report
+
+Return:
+
+- PR URL
+- Title used
+- Local typecheck/build status (if you ran them)
+- A note for the reviewer if anything in the checklist is left unchecked