Default the manifest to index.mf and keep it out of its own listing (closes #100)

mfer gen now writes index.mf instead of .index.mf, the name fetch
requests and the README calls the standard filename. Given a
directory, check, freshen, list and export look only for index.mf;
.index.mf is no longer recognized. Because index.mf is not hidden, gen
and freshen now leave the manifest they write out of its own listing by
file identity (os.SameFile), so it is recognized however its path is
spelled, through a symlink included, while an ordinary file of the same
name elsewhere in the tree is still listed.

Model: opus-5-5

.gitignore

@@ -8,7 +8,7 @@ vendor.tzst
 modcache.tzst
 
 # Generated manifest files
-.index.mf
+/index.mf
 
 # Secrets
 .env