Write a real README for herb-embedded (herb-embedded-xbi)

README.md

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 # herb-embedded
+
+Lints ERB templates with [Herb](https://herb-tools.dev)'s actual rule set — 106 built-in
+rules spanning accessibility, Action View, ERB, HTML, SVG, and Turbo markup, plus autofix —
+as a plain Ruby dependency. Nothing shells out to Node while it runs.
+
+Herb itself is JS-first: `herb lint` shells out to `npx @herb-tools/linter` under the hood,
+and the other Ruby-side integrations either do the same (`pronto-herb`) or aren't implemented
+yet (`rubocop-herb`). That's a dead end for a Rails app that has deliberately kept Node off
+its stack — importmap-rails plus the precompiled `tailwindcss-ruby` binary being the usual
+reason. herb-embedded gets around it by vendoring Herb's real TypeScript rule engine and
+running it inside an embedded JS engine (V8, via mini_racer) from within your Ruby process —
+so you get the actual upstream rules, not a Ruby reimplementation of them, without installing
+Node anywhere. Full rationale and scope in [CHARTER.md](CHARTER.md).
+
+## Install
+
+Not yet published to rubygems.org — add it straight from GitHub:
+
+```ruby
+# Gemfile
+gem "herb-embedded", git: "https://github.com/defmethodinc/herb-embedded.git"
+```
+
+```bash
+bundle install
+```
+
+Or build and install it locally from a clone:
+
+```bash
+git clone https://github.com/defmethodinc/herb-embedded.git
+cd herb-embedded
+bundle install
+bundle exec rake build
+gem install pkg/herb-embedded-*.gem
+```
+
+## Usage
+
+Lint every file your config (or the defaults) picks up:
+
+```bash
+bundle exec herb-lint-rb
+```
+
+With no arguments, herb-lint-rb discovers files itself, from `.herb.yml`'s
+`files.include`/`files.exclude` globs (default: `**/*.html.erb` and `**/*.herb` under the
+current directory) — it does not accept a directory as an argument. Pass explicit file paths
+instead to lint a subset:
+
+```bash
+bundle exec herb-lint-rb app/views/widgets/_card.html.erb app/views/widgets/_footer.html.erb
+```
+
+Exit codes: `0` clean, `1` offenses found at or above the fail level, `2` a config/flag error.
+
+## CLI flags
+
+| Flag | Effect |
+| --- | --- |
+| `--fix` | Apply safe autocorrections |
+| `--fix-unsafely` | Apply unsafe autocorrections too (implies `--fix`) |
+| `--format FORMAT` | `detailed` \| `simple` \| `json` \| `github` (default: `detailed`) |
+| `--fail-level LEVEL` | `error` \| `warning` \| `info` \| `hint` — minimum severity that fails the run (default from `.herb.yml`'s `linter.fail_level`, itself defaulting to `error`) |
+| `--only RULE` | Run only this rule (repeatable) |
+| `--init` | Write a starter `.herb.yml` pinning the currently bundled linter version |
+| `--version` | Print the herb-embedded and bundled `@herb-tools/linter` versions |
+| `-h`, `--help` | Show help |
+
+## Configuring `.herb.yml`
+
+`bundle exec herb-lint-rb --init` writes a starter file:
+
+```yaml
+version: "0.10.3"
+files:
+  include:
+    - "**/*.html.erb"
+    - "**/*.herb"
+  exclude: []
+linter:
+  fail_level: error
+  rules: {}
+```
+
+- `version` records which bundled linter version the file was generated against; it's
+  informational, not enforced.
+- `files.include` / `files.exclude` are glob lists (relative to the directory `herb-lint-rb`
+  runs from) that control which files a no-argument run discovers.
+- `linter.fail_level` sets the default minimum severity that produces a non-zero exit code
+  (overridable per-run with `--fail-level`).
+- `linter.rules.<rule-name>.enabled: false` disables a built-in rule:
+
+  ```yaml
+  linter:
+    rules:
+      html-no-self-closing:
+        enabled: false
+  ```
+
+Only these keys are read; any other key from upstream `@herb-tools/config`'s schema is parsed
+but ignored, not an error.
+
+## Custom rules
+
+Drop `.mjs` files under `.herb/rules/`. The only supported import is named imports from
+`@herb-tools/linter` (rewritten at load time to pull from the already-loaded bundle — nothing
+else can be imported, and an unsupported import fails loudly instead of producing a runtime
+`ReferenceError`):
+
+```js
+// .herb/rules/my_custom_rule.mjs
+import { ParserRule } from "@herb-tools/linter";
+
+export default class MyCustomRule extends ParserRule {
+  static ruleName = "my-custom-rule";
+
+  check(result, context) {
+    return [this.createOffense("Custom offense!", result.value.location)];
+  }
+}
+```
+
+A custom rule whose `ruleName` matches a built-in rule replaces it for that run (and prints a
+warning to stderr).
+
+## Learn more
+
+[CHARTER.md](CHARTER.md) covers what this gem is, who it's for, what "good" means here, and
+what's explicitly out of scope — read that before proposing a change to its shape.
+[CONTRIBUTING.md](CONTRIBUTING.md) covers development setup.