[agents-md] Address review sweep nits
`requireNextProjectDir` JSDoc corrected — the function throws a plain
`Error`, not `BadInput`, because `lib/` can't import `BadInput` from
`bin/shared.ts` without inverting the layering. The codemod `bin/`
callers catch and rethrow as `BadInput`; `bin/upgrade.ts` now wraps
its call too (defense in depth — should never fire in practice, but
keeps a future reorder from leaking a stack trace).
`upsertAgentRulesBlock` now detects the existing file's line-ending
style and matches it when injecting the managed block. Previously,
stomping `\n` into a CRLF file on Windows produced mixed EOLs that
confuse editors and `.gitattributes` normalization. `stripLegacyAgentRulesBlock`
loops until no legacy block remains (defensive — if a file ever
acquired multiple legacy blocks via manual paste, both get stripped)
and takes the same `eol` so the re-joined seam doesn't introduce
mixed EOLs. `writeBundledDocsAgentFiles` doc now notes the
two-file create branch is non-atomic — re-running cleanly recovers,
so we accept the transient inconsistency rather than add cleanup
bookkeeping for a cold-path failure.
Test fixture dirs now use `fs.mkdtempSync` instead of
`Date.now() + Math.random()` so parallel workers can't collide. Two
new e2e tests cover the CRLF preservation path and multi-legacy-block
stripping.
