feat: add no-useless-echo rule

[nzakas]

Adds shell/no-useless-echo, which mirrors ShellCheck SC2116: cmd $(echo foo) is just cmd foo.

Behavior

• Reports a command substitution ($(...) or backticks) whose body is exactly one echo command, not negated and with no redirects.
• The check doesn't look at echo's arguments, so $(echo -n foo) is reported too.
• Not reported: substitutions with more than one statement ($(echo a; echo b)), pipelines ($(echo foo | tr a-z A-Z)), or redirected echoes.
• No autofix: removing the substitution can change quoting and word splitting.
• Recommended config: "error".

Documentation

Adds docs/rules/no-useless-echo.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

8 RuleTester cases and 5 documentation checks (244 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 3b6181d and 29df217.

📒 Files selected for processing (6)

• README.md
• docs/rules/no-useless-echo.md
• src/index.spec.ts
• src/index.ts
• src/rules/no-useless-echo.spec.ts
• src/rules/no-useless-echo.ts

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[Copilot]

Copilot review overview

🟢 Approval recommended

The implementation and integration are sound; only a minor test-coverage gap remains.

Review effort: Balanced
Findings: 1

Open (1)

• Add test for documented negated-command exception · New

What changed in this PR

Adds a ShellCheck-inspired rule detecting redundant echo command substitutions.

Changes:

• Implements and tests bash/no-useless-echo.
• Enables the rule in the recommended configuration.
• Adds rule documentation and README registration.

File	Description
src/​rules/​no-useless-echo.ts	Implements detection logic.
src/​rules/​no-useless-echo.spec.ts	Adds RuleTester coverage.
src/​index.ts	Registers and enables the rule.
src/​index.spec.ts	Verifies rule exposure.
README.md	Adds the rule to the catalog.
docs/​rules/​no-useless-echo.md	Documents behavior and usage.

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[nzakas]

@copilot address review comments. Be sure to add tests.

[Copilot]

@copilot address review comments. Be sure to add tests.

Added the negated echo regression case and verified it with the targeted test suite. Commit: 7b6d5b7.