[Codemod] Fix agents-md on Windows (#89319)
Fixes #89240
The `agents-md` codemod failed on Windows because it assumed forward
slashes in file paths. This caused the doc tree builder to skip files,
resulting in an empty index being injected into `AGENTS.md`.
The path handling code used string operations like `.split('/')` and
`.endsWith('/index.mdx')` which don't work with Windows backslashes. Now
uses regex patterns that match both separators: `/[/\\]/` for splitting
and `!/[/\\]index\.mdx$/` for filtering. Also normalizes all paths to
forward slashes in the output.
While here, switched from `execSync` to `execa` for git commands. This
is the pattern already used elsewhere in the codemod package
(`upgrade.ts`, `handle-package.ts`) and passes arguments as an array
instead of shell string interpolation.
Tests were replaced with e2e tests that run the full CLI and include a
cross-platform path normalization test case.
