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 leave out of their own listing the manifest they write and
a temp file an interrupted run left beside it, matched by file
identity (os.SameFile) however the path is spelled. The scanner takes
these as a plain ExcludePaths list; manifestTempPath alone names the
temp file, for both writes, both skips and the tests.

Model: opus-5-5

.gitignore

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