feat: add no-backticks rule

[nzakas]

Adds shell/no-backticks, which mirrors ShellCheck SC2006: use $(...) instead of legacy backtick command substitution.

Behavior

• Reports every backtick command substitution, including ones inside double quotes. Nested backticks are reported separately.
• Autofix: rewrites `cmd` to $(cmd). The fix is skipped when the substitution contains a backslash or a nested backtick, because escaping rules differ inside backticks and a textual rewrite could change the meaning.
• Recommended config: "error".

Also in this PR

This is the first rule in the stack, so besides registering the rule it:

• adds the README's Rules table and a manual-configuration example;
• adds a generic index.spec.ts check that every rule has docs, messages, and schema metadata, and that meta.docs.url points at the rule's file in docs/rules;
• adds tests/rule-docs.test.ts, which checks every registered rule's doc: the title and description match the rule's metadata, the standard sections are present, every incorrect example is reported by the rule, and every correct example produces no messages;
• creates tests/autofix.test.ts.

Every rule PR registers its rule in the same three places: src/index.ts (the rules map and the recommended config), the rule list in src/index.spec.ts, and the README table. Each rule PR also adds the rule's doc in docs/rules.

Documentation

Adds docs/rules/no-backticks.md, following the format of the @eslint/json, @eslint/css, and @eslint/markdown rule docs: description, background, rule details with incorrect and correct examples, options, when not to use it, and the ShellCheck reference. The rule's meta.docs.url points at that file, and its README table entry links to it.

Testing

• 9 RuleTester cases, including fixed output, a no-fix case, and exact report positions.
• An end-to-end verifyAndFix test.
• The metadata check.
• 5 documentation checks from the new docs test.

170 tests total; build, lint, and format checks pass.

🤖 Generated with Claude Code

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📥 Commits

Reviewing files that changed from the base of the PR and between dde227f and ee025fe.

📒 Files selected for processing (8)

• README.md
• docs/rules/no-backticks.md
• src/index.spec.ts
• src/index.ts
• src/rules/no-backticks.spec.ts
• src/rules/no-backticks.ts
• tests/autofix.test.ts
• tests/rule-docs.test.ts

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[Copilot]

Copilot review overview

🟢 Approval recommended

The implementation, conservative fix behavior, registration, documentation, and tests are consistent and complete.

Review effort: Balanced
Findings: None

What changed in this PR

Adds the first Bash lint rule, discouraging legacy backtick command substitution.

Changes:

• Implements and registers bash/no-backticks with safe autofixing.
• Adds rule, integration, metadata, and documentation tests.
• Documents configuration and rule behavior.

File	Description
src/​rules/​no-backticks.ts	Implements reporting and autofixing.
src/​rules/​no-backticks.spec.ts	Tests rule behavior and fixes.
src/​index.ts	Registers and recommends the rule.
src/​index.spec.ts	Verifies registration and metadata.
tests/​autofix.test.ts	Tests end-to-end autofixing.
tests/​rule-docs.test.ts	Validates rule documentation examples.
docs/​rules/​no-backticks.md	Documents the rule.
README.md	Adds configuration and rules guidance.

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