fix: refuse unexpected gitlinks staged by git add

[EndBug]

Summary

• After git add, inspect git diff --cached --raw and fail if any new mode-160000 gitlink would be committed (embedded nested .git).
• Allow updates that stay 160000→160000 so existing submodule SHA bumps still work.
• Document the behavior in the README and cover the parser/assert helpers with unit tests.

Test plan

• npm test passes (parser fixtures: new gitlink, mode change into gitlink, submodule bump, mixed/empty)
• Manual PoC: nested .git under the working tree + default add: '.' fails the action with remediation hints instead of committing a gitlink
• Existing submodule path update (160000→160000) still commits successfully
• CI confirms committed lib/index.js matches a clean rebuild

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📥 Commits

Reviewing files that changed from the base of the PR and between 75038f8 and ee6ea87.

📒 Files selected for processing (5)

• README.md
• lib/index.js
• src/main.ts
• src/util.ts
• test/util.test.ts

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[Copilot]

Pull request overview

This PR adds a safety check to prevent the action from committing newly introduced gitlinks (mode 160000) after git add, which can happen when an embedded/nested .git directory is staged and would otherwise be recorded as a submodule-like reference.

Changes:

• Add parsing + assertion helpers to detect newly introduced mode-160000 entries in git diff --cached --raw.
• Enforce the check in the add() flow so the action fails fast when unexpected gitlinks are staged.
• Document the behavior and add unit tests for the parser/assert helpers.

Reviewed changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 3 comments.

File	Description
src/util.ts	Adds gitlink detection/parsing and a failing assertion with remediation hints.
src/main.ts	Runs the cached raw diff check after git add and fails on unexpected gitlinks.
test/util.test.ts	Adds unit tests for the new gitlink parser and assertion helper.
README.md	Documents the new “refuse new gitlinks” behavior and remediation steps.

Suppressed comments (2)

src/util.ts:190

• The remediation hint uses git rm --cached ${p} without a -- path separator. If a path begins with -, the pasted command can be interpreted as an option instead of a path. Adding -- makes the suggested command safe for all path names.

  const listed = paths.map(p => `  - ${p}`).join('\n');
  const rmHints = paths.map(p => `  git rm --cached ${p}`).join('\n');
  throw new Error(
    `Refusing to commit unexpected gitlink(s) (embedded git repository staged as mode 160000):\n${listed}\n` +
      'Git records a nested .git directory as a gitlink, not as its files. ' +
      `Remove the nested .git directory, or unstage the path(s) with:\n${rmHints}`,

test/util.test.ts:235

• This assertion hard-codes the remediation hint without the -- path separator. If the hint is updated to git rm --cached -- <path> (recommended for safety), the test should assert the safer form to match.

    );
    expect(() => assertNoUnexpectedGitlinks(['evil_nested_repo'])).toThrow(
      /git rm --cached evil_nested_repo/,
    );

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