chore: add .gitattributes to normalize line endings to LF

[devorun]

Summary

The repository ships no .gitattributes, so on Windows (git core.autocrlf=true, the default) a fresh clone checks out text files with CRLF. pnpm test then fails out of the box: the skill-sync drift guard in packages/agent-core asserts content.startsWith("---\nname: ..."), which is false when the SKILL.md files are CRLF.

Adding a standard .gitattributes normalizes text files to LF on checkout, so a Windows contributor's working tree matches what the tests (and the Linux CI) already expect.

Change

• .gitattributes — * text=auto eol=lf

A single line. git add --renormalize . stages nothing else (the index is already LF), so there is no mass line-ending churn. No .bat / .cmd / .ps1 files exist that would need a eol=crlf exception, and no committed fixtures or snapshots depend on CRLF.

Testing

• Before: on Windows, pnpm --filter @sapiom/agent-core test fails two skill-sync assertions (has a canonical source with the … trigger frontmatter).
• After renormalizing the working tree to honor the new attribute, the SKILL.md files check out as LF and skill-sync passes 7/7.
• No published package changes, so no changeset is included.