docs: add AGENTS.md for AI coding agents

AGENTS.md

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+# AGENTS.md
+
+Guidance for AI coding agents in this repo. Human contributors: see the
+[Contributing](./README.md#contributing) section of the README.
+
+This is `attr-accept`: a single function, `accept(file, acceptedFiles)`, that checks whether a file
+matches an HTML `<input type="file">` `accept` attribute value (MIME types and/or file extensions).
+It is a tiny, published npm library written in TypeScript with no runtime dependencies; keep it that
+way - the whole point is to stay small.
+
+The toolchain is the Rust-based oxc stack: [oxlint](https://oxc.rs/) to lint,
+[oxfmt](https://oxc.rs/) to format, [tsdown](https://tsdown.dev/) (Rolldown + oxc) to build and emit
+declarations, and [Vitest](https://vitest.dev/) to test; TypeScript (`tsc --noEmit`) type-checks and
+`size-limit` budgets the bundle. There is no Babel, ESLint, Prettier, or Rollup; do not reintroduce
+them.
+
+## Workflow
+
+- Clarify the design before implementing. For anything non-trivial, agree on the approach first.
+- One unit of change per commit. Never mix unrelated changes. Present the change for review before
+  committing.
+- Every change ships with tests. Run local CI before calling it done, and do not claim it passes
+  without running it.
+- Verify against the code and the tools: read before you answer, run before you assert.
+
+Local CI (must be green before review):
+
+```shell
+npm run type-check      # tsc --noEmit
+npm run lint            # oxlint
+npm run lint:type-aware # oxlint --type-aware
+npm run format:check    # oxfmt --check
+npm run build           # tsdown -> dist/
+npm run test:cov        # vitest with coverage
+npm run size            # size-limit bundle budget
+```
+
+The prek git hooks (installed via `npm install`) auto-run oxfmt and oxlint on staged code and
+validate the commit message, but they do not run type-check, the build, or the tests, and they do
+not format Markdown. Run the commands above yourself, and run `npm run format` after editing
+docs/Markdown or CI's `format:check` will fail on it.
+
+## Writing: code, comments, docs, commits
+
+- Concise and to the point. No fluff. Explain the non-obvious; do not narrate the obvious.
+- ASCII only. No em-dash and no `--`; write `-`. Use `->` not the arrow glyph, `!=` not the
+  not-equal glyph, and so on.
+- Comments justify _why_, not _what_. Delete any comment that restates the code.
+- Formatting is not a matter of taste: oxfmt owns it. Run `npm run format` rather than
+  hand-formatting. House style (`.oxfmtrc.json`) is double quotes, two-space indent, semicolons, no
+  trailing commas, no bracket spacing (`{a, b}`), arrow parens omitted when possible, and a
+  120-column print width.
+
+## Commits
+
+- [Conventional Commits](https://www.conventionalcommits.org/); the type set is enforced by a
+  commit-msg hook and consumed by semantic-release. Write the subject in the present tense,
+  imperative voice: `fix: match extensions case-insensitively`, not `fixed` or `fixes`.
+- `feat:`/`fix:`/`perf:` cut a release; `feat!:` or a `BREAKING CHANGE:` footer cuts a major.
+  `chore:`/`ci:`/`docs:`/`test:`/`refactor:`/`style:`/`build:` do not. Pick the type with that in
+  mind.
+- Keep the body minimal, or omit it. A good subject plus the diff is usually enough; add a body only
+  for what the code cannot show (why, a trade-off, a non-obvious consequence). Never restate the
+  change or narrate the diff.
+- Disclose AI with an `Assisted-by: Claude:claude-opus-4-8` trailer. Never `Co-Authored-By`, and
+  never add a human's `Signed-off-by`.
+
+## Tests
+
+- Unit tests live beside the source as `src/*.spec.ts` and run under Vitest in the `node`
+  environment (the function is pure and does not touch the DOM).
+- Cover the matching rules directly: wildcard MIME types (`image/*`), exact MIME types, extensions
+  (`.png`), array vs comma-delimited input, and the "accept when there is nothing to match against"
+  short-circuits. Table-driven cases keep this readable.
+- Coverage must not drop. New code ships with tests that hold or raise it. Measure with
+  `npm run test:cov`.
+
+## Code conventions
+
+- Source is TypeScript (ESM); no JSX. The published API is exactly what `src/index.ts` exports: the
+  default `accept` function and the `FileWithType` type. Keep the README usage examples in step with
+  any public change.
+- Stay dependency-free and small. The `size-limit` budget in `package.json` fails the build if the
+  bundle grows past its limit; treat that as a design constraint, not an obstacle to raise.
+
+## Build and publish
+
+- `npm run build` bundles `src/index.ts` with tsdown into `dist/` (ESM `.js`, CJS `.cjs`, and the
+  `.d.ts` emitted from source). Do not hand-edit anything in `dist/` - it is generated.
+- What ships to npm is the `files` allowlist in `package.json` (`dist` and `src`, minus specs); keep
+  it accurate.
+- Releases are automated by semantic-release from the commit history; the repo version stays
+  `0.0.0-development` and is set at publish time. Never bump the version by hand. Runtime is Node
+  `>= 22` (`engines`).
+
+## CI workflows
+
+- GitHub Actions live in `.github/workflows` (`test.yml` runs lint/format/types, the test matrix,
+  and the size check; `release.yml` publishes). Write the workflow `name:`, every job name, and
+  every named step in Sentence case (match the existing files).
+- Dependabot groups patch/minor bumps and auto-merges patches on green CI (`.github/dependabot.yml`,
+  `dependabot-auto-merge.yml`). Auto-merge trusts CI, so any check that must gate a dependency bump
+  has to run in CI.
+- Keep workflows minimal and scoped to one purpose; prefer the built-in `GITHUB_TOKEN` over a
+  personal access token.

CLAUDE.md

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+@AGENTS.md