Fix Windows artifact refresh in write_file_atomic

[Ayush7Ranjan]

Summary

Fixes Windows artifact persistence for index_repository with
persistence=true.

On Windows, .codebase-memory/graph.db.zst was created on the
first run, but subsequent runs did not refresh/replace the
artifact. The root cause was src/pipeline/ artifact.c::write_file_atomic() using rename(tmp, path),
which does not reliably replace an existing destination file
on Windows.

This PR updates the atomic replace path so:

• Windows uses MoveFileExA(tmp, path, MOVEFILE_REPLACE_EXISTING | MOVEFILE_WRITE_THROUGH)
• POSIX keeps the existing rename(tmp, path) behavior

Verification

Built on Windows with MSYS2:

C:\msys64\usr\bin\bash.exe -lc "export HOME=/tmp; export
PATH=/mingw64/bin:/usr/bin:$PATH; cd /c/Users/aayus/OneDrive/
Desktop/opensource/codebase-memory-mcp; scripts/build.sh
CC=gcc CXX=g++"

Ran the Issue #400 reproduction twice with:

$env:CBM_CACHE_DIR = ".\.codebase-memory"
$env:CBM_PERSIST_EXPORTS = "true"

Result:

First run:
artifact_present=true
graph.db.zst exists=true
size=7586364
lastWriteUtc=2026-06-17T18:58:02.0340409Z

Second run:
artifact_present=true
graph.db.zst exists=true
size=9914129
lastWriteUtc=2026-06-17T18:58:07.9280511Z

timestamp_changed=True

## Fixes

Fixes #400