test: add CI coverage for grouped skip-component precedence (Issue #32)

[justinthelaw]

Motivation

• Add targeted CI coverage for high-risk interaction paths between grouped components and their subcomponents to prevent regressions in removal precedence.
• Ensure skip-components normalization (whitespace and case) and max vs custom precedence are exercised by CI so the action's destructive behavior remains predictable.

Description

• Added three focused jobs to /.github/workflows/test.yml: max-skip-cached-node-precedence, max-skip-firefox-precedence, and custom-grouped-subgroup-precedence, each capturing pre-action state, exercising the action with normalized/mixed-case skip-components or grouped toggles, and verifying expected post-action state.
• Verification checks include preserving Node toolcache when skip-components: cached-node is used, preserving Firefox when skip-components: firefox is used, and asserting grouped precedence scheduling via /tmp/total_ops when both remove-browsers and remove-firefox are requested in custom mode.
• Updated README.md with a CI-verified interaction guarantees section documenting the behaviors protected by the new tests.
• Updated maintainer guidance in docs/CONTRIBUTING.md and repository notes in AGENTS.md to require/reflect interaction coverage for grouped/subgroup precedence and skip-components normalization.

Testing

• Parsed YAML for action.yml and /.github/workflows/test.yml with ruby -e 'require "yaml"; ...' which succeeded, confirming workflow and action syntax is valid.
• Ran git diff --check which reported no whitespace or diff errors.
• Attempted pre-commit run --all-files but it was not executed in this environment because pre-commit is not installed (manual or CI pre-commit checks still required before merging).

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Codex Task