fix(tools): create new files with the platform-native line ending

[sonderhq]

Problem

A newly created file was written with LF endings whenever the model's content carried no CRLF:

const lineEndings = existingMetadata?.lineEndings ?? (input.content.includes("\r\n") ? "CRLF" : "LF");

Created files have no existing EOL to preserve, so on Windows they ended up with LF while native tooling (and the files a user's editor creates) use CRLF. Existing files were unaffected — their recorded line endings already win, and reading already has detectLineEndings() for exactly that purpose.

Fix

platformLineEnding() in packages/core/src/common/file-utils.ts supplies the default for created files. It mirrors detectLineEndings() on the read side and takes the platform EOL as an injectable parameter so both branches are testable off-Windows. On LF platforms the behaviour is unchanged (no-op).

Tests

New packages/core/src/tests/write-handler-line-endings.test.ts:

• platformLineEnding("\r\n") / platformLineEnding("\n") → CRLF / LF (both branches, any host platform);
• a created file matches the platform-native ending (Windows CI exercises the CRLF branch for real);
• an existing CRLF file keeps its endings when rewritten with LF content (no regression to the preservation path).

Verification:

• node src/tests/run-tests.mjs (core): 385 tests, 383 pass, 0 fail (2 skipped);
• npm run typecheck, npm run lint, prettier --check clean; pre-commit hook passed.