docs: add AGENTS.md with CLAUDE.md symlink

Author: mscolnick

AGENTS.md

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+# codemirror-sql
+
+CodeMirror 6 extension for SQL: real-time linting, gutter indicators, hover tooltips, and schema-aware autocomplete (DuckDB, BigQuery, Dremio dialects). Published to npm as `@marimo-team/codemirror-sql` and used by marimo's editor.
+
+## Development
+
+```bash
+pnpm install --ignore-scripts --frozen-lockfile  # CI install
+pnpm test              # vitest
+pnpm run test:browser  # vitest browser tests (Playwright)
+pnpm run lint          # oxlint --fix (autofix.ci runs this on PRs)
+pnpm exec oxlint       # non-mutating lint CI enforces
+pnpm run typecheck     # tsc --noEmit
+pnpm run demo          # vite build of demo/
+```
+
+- Browser tests need Playwright browsers installed first: `pnpm exec playwright install`.
+- Release: `pnpm run release` (pnpm version) bumps + tags; pushing a `v*` tag triggers release.yml, which publishes to npm via OIDC.

CLAUDE.md

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+AGENTS.md