chore: add .gitattributes to normalize line endings

[MohammedAlkindi]

On Windows, git's default core.autocrlf=true checks out CRLF, and the contributor commands in CONTRIBUTING.md then fail on a clean clone.

Before (fresh clone of main at 3f854a8, measured 2026-08-21, Windows 11, stock git config):

$ bun run format:check
[warn] Code style issues found in 191 files. Run Prettier with --write to fix.
error: script "format:check" exited with code 1

bun test: 882 pass, 49 fail.

After (same commit, with this file):

$ bun run format:check
Checking formatting...
All matched files use Prettier code style!

bun test: 885 pass, 46 fail.

Prettier defaults to endOfLine: "lf" and .prettierrc is {}, which accounts for the 191 files. Three tests fail only under CRLF and pass with this change:

• action metadata > should expose the conclusion output from the run step
• base action README > should document every input declared in the action metadata
• integration tests > formats real conversation data correctly

Each matches LF-terminated content, for example the /^ conclusion:\n...$/m assertion at test/action-metadata.test.ts:11. The remaining 46 failures are pre-existing Windows issues (path separators, POSIX permission bits, symlink privilege) and are untouched by this change.

A few things I checked before opening this:

• All tracked blobs are already LF, so there is no renormalization. git add --renormalize . stages zero changes, and the diff is one new file.
• * text=auto on its own does not fix this. It governs normalization on commit, not checkout, so the working tree stays CRLF. eol=lf is the minimal line that works.
• Every CI job runs on ubuntu-latest, where this is a no-op. Checkouts with and without the file are byte-identical under core.autocrlf=false.
• The repo tracks no binary files and no .bat, .cmd, or .ps1 files, so nothing needs CRLF preserved.
• This does not reach claude-code-base-action. sync-base-action.yml triggers on base-action/** and copies only that directory, so a root file is neither triggered nor mirrored. Glad to add base-action/.gitattributes as well if you want the mirror covered, but I have left it out to keep this to one change.

AI assistance: I used Claude Code for this investigation. What I verified myself on my own Windows 11 machine, rather than taking on trust: the before and after format:check exit codes and the 191 file count, the three CRLF-only test failures, that * text=auto alone still leaves action.yml with 459 CR bytes in the working tree while * text=auto eol=lf leaves 0, and that every tracked blob is already stored LF.