feat: add pkg/mnt — mount operations for Core framework
core.mnt provides zero-dep mount operations:
- mnt.FS(embed, "subdir") — scoped embed.FS access (debme pattern)
- mnt.Extract(fs, targetDir, data) — template directory extraction (gosod/Install pattern)
Template extraction supports:
- Go text/template in file contents (.tmpl suffix)
- Go text/template in directory and file names ({{.Name}})
- Ignore files, rename files
- Variable substitution from any struct or map
Based on leaanthony/debme (70 lines) + leaanthony/gosod (280 lines),
rewritten as single zero-dep package. All stdlib, no transitive deps.
8 tests covering FS, Sub, ReadFile, ReadString, ReadDir, Extract.
Co-Authored-By: Virgil <virgil@lethean.io>