Remove committed .index.mf and add to .gitignore (#52)

Closes #49

`.index.mf` is a generated manifest file produced at runtime by `mfer gen`. It was committed to the repo but should not be tracked in version control.

Changes:
- `git rm .index.mf` to remove it from tracking
- Added `.index.mf` to `.gitignore` to prevent accidental re-commits

Co-authored-by: user <user@Mac.lan guest wan>
Reviewed-on: #52
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

.gitignore

@@ -6,5 +6,8 @@
 vendor.tzst
 modcache.tzst
 
+# Generated manifest files
+.index.mf
+
 # Stale files
 .drone.yml