ArtifactFS 1.0.0-rc.4

Release candidate patch for ArtifactFS 1.0.

This supersedes 1.0.0-rc.3 and includes the git status performance work from PR #26.

Changes since 1.0.0-rc.3:

• Speed up git status on ArtifactFS mounts by avoiding full clean worktree walks through FUSE.
• Add ReadDirPlus support so directory listings can return child attributes without per-entry lookup.
• Configure a repo-local fsmonitor hook for managed gitdirs and enable fsmonitor on macFUSE with fsmonitor.allowRemote=true.
• Report exact dirty overlay paths from the fsmonitor hook so a single modified file does not dirty a whole subtree.
• Mark index entries fsmonitor-clean only after HEAD generation and runtime state are updated.
• Fix fsmonitor test portability.

Verification:

• GitHub PR #26 checks passed: Build & Test, CodeQL, and Semgrep.
• Local release validation from merged origin/main: go build ./cmd/artifact-fs, go vet ./..., go test ./..., and go run golang.org/x/tools/go/analysis/passes/modernize/cmd/modernize@latest -fix ./... && git diff --exit-code.