feat: add safe dotenv linting

[zeitlinger]

Summary

• add dotenv-linter checking for explicit .env-style files
• add serialized fixing without backup files or secret-bearing output
• add generated documentation, tool pins, and focused end-to-end cases

Split from #430 so the dotenv-linter integration can be reviewed independently.

Validation

• mise run generate
• mise run lint:fix
• mise exec -- cargo test

[zeitlinger]

Pre-release consumer validation is running in open-telemetry/opentelemetry-java-examples#1226. The real dotenv-linter run found and fixed an ordering issue in otlp/docker/.env; the focused lint, full Flint lint, and Gradle assemble pass locally.

[Copilot]

Pull request overview

Adds first-class dotenv-linter integration to Flint so explicit .env-style files can be linted (and optionally fixed) without scanning unrelated config files, and wires the new tool through generated docs, pins, Renovate tracking, and end-to-end cases.

Changes:

• Register a new built-in dotenv-linter check scoped to .env, .env.*, and *.env, including --fix support.
• Add focused e2e cases for clean and auto-fix behavior.
• Update generated docs/overview tables and add tool pin + Renovate tracking for dotenv-linter.

Reviewed changes

Copilot reviewed 14 out of 14 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tests/cases/dotenv-linter/clean/test.toml	Adds a clean e2e case for running dotenv-linter.
tests/cases/dotenv-linter/clean/files/mise.toml	Pins dotenv-linter in the clean case fixture.
tests/cases/dotenv-linter/clean/files/.env	Provides a sample valid .env file for the clean case.
tests/cases/dotenv-linter/auto-fix/test.toml	Adds an auto-fix e2e case using a fake dotenv-linter binary.
tests/cases/dotenv-linter/auto-fix/files/mise.toml	Pins dotenv-linter in the auto-fix case fixture.
tests/cases/dotenv-linter/auto-fix/files/.env	Provides an invalid .env input to be fixed in the auto-fix case.
src/registry/checks.rs	Registers the new dotenv-linter check (patterns, commands, docs links).
src/main.rs	Updates the linters table fixture to include the new check entry.
README.md	Updates the generated Files/Formats table to include Dotenv.
mise.toml	Pins dotenv-linter for contributor tooling via mise.
docs/linters.md	Adds generated documentation for dotenv-linter and updates the Files/Formats table.
default.json	Includes dotenv-linter in the default tracked tools list.
.github/renovate.json5	Adds dotenv-linter to Renovate-managed dependencies.
.github/renovate-tracked-deps.json	Tracks dotenv-linter releases via GitHub releases datasource.

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