Background

Windows isn't currently CI-tested or formally documented as supported.
This issue proposes promoting Windows from implicitly unsupported to
CI-tested, motivated by PR #633 — which fixed 5 tests (one of which
propagates to a 14-test class via its fixture) that hit
OSError: [WinError 1314] (client does not hold the required privilege)
because they call os.symlink / Path.symlink_to. CI never noticed
because every job in .github/workflows/ci.yml runs on ubuntu-latest.

There's no broken cross-platform promise to fix here — README.md,
packages/memtomem/README.md, and CONTRIBUTING.md don't claim Windows
support today. The relevant prior art is the implicit POSIX-only stance
of the mm: command not found README tip (which mentions ~/.local/bin
on macOS/Linux only). What this issue proposes is a deliberate step
toward supporting Windows by adding the CI signal first.

There are likely other latent Windows-specific issues hiding behind the
ubuntu-only blind spot — see "Likely first-run failures" below.

Proposal

Add windows-latest (and probably macos-latest while we're at it) to
selected jobs in .github/workflows/ci.yml. The repo currently has 6
jobs; per-job recommendations:

Concretely:

test:
  strategy:
    fail-fast: false
    matrix:
      os: [ubuntu-latest, windows-latest, macos-latest]
  runs-on: ${{ matrix.os }}

Notes on cost / latency

Actions minutes are free and unlimited for public repos (this repo
is public), so the 2x/10x billing multipliers for Windows/macOS don't
apply here. The real concern is PR feedback wallclock: Windows
runners take longer to provision and Python runs measurably slower on
Windows. Adding 6 jobs × 3 OSes = 18 jobs may slow PR turnaround even
when the bill is zero. The per-job matrix above keeps the fast-feedback
path (lint + typecheck) on a single OS for that reason.

Developer Mode default

GitHub-hosted windows-latest runners (currently windows-2022) have
Developer Mode enabled by default out of the box — verified against
the actions/runner-images manifest, has been true since 2021. So PR
#633's requires_symlinks marker will run the symlink tests on
Windows CI, not skip them, which is what we want.

Likely first-run failures

First Windows CI run will likely be red on more than just symlinks —
that's the point of adding the runner. Known surfaces to expect:

• Hardcoded POSIX paths in tests. Path("/tmp/test.md") and
  similar appear in 9 test files: test_reranker_fastembed.py,
  test_uninstall_cmd.py, test_ask.py, test_indexing_engine.py
  (×4 sites), test_runtime_paths.py. Most are metadata-only
  (source_file=Path(...) passed without being opened), so some may
  pass on Windows by accident — worth triaging real bugs vs. cosmetic.
• POSIX file-mode bits. tests/test_runtime_paths.py does
  mode=0o700 mkdir + mode=0o755 checks (4 sites). NTFS doesn't
  translate POSIX modes cleanly; the whole XDG_RUNTIME_DIR test class
  may need a Windows skip + a Windows-equivalent test.
• mm web PID-file handling. _runtime_paths.py has POSIX-first
  XDG fallback to tempfile.gettempdir(), but the Windows fallback
  branch isn't exercised on CI today.
• Line endings (CRLF). git config core.autocrlf true is the
  Windows default. Tests doing exact-byte comparison
  (e.g. assert (dest / "SKILL.md").read_bytes() == b"# foo skill\n"
  in test_install_skill_copies_tree) will break if checkout
  normalizes to CRLF. .gitattributes doesn't pin text eol=lf
  today — would need either pinning or test rewrite.
• uv tool install shim path. README's mm: command not found
  troubleshooting tip mentions ~/.local/bin (POSIX-only) — the
  Windows shim path differs (%APPDATA%\uv\tools\...). Once Windows
  is CI-tested, the user-facing install flow is also worth covering.
• Subprocess invocations that assume a POSIX shell.

Acceptance criteria

• .github/workflows/ci.yml runs the test job on
  windows-latest (and macos-latest).
• First-pass Windows-specific failures triaged: either fixed, or
  marked with the appropriate platform skip + a tracking
  sub-issue.
• CI green on all matrix cells.
• README / CONTRIBUTING reflects which platforms are now CI-tested
  (this is the moment Windows becomes a documented promise).

References

• PR test(windows): auto-skip symlink tests when filesystem can't symlink #633 — the symlink-skip workaround that motivates this issue.

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