refactor: introduce virtual file system as a central router for source and target data

content/treehouse/vfs.tree

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+%% title = "treehouse virtual file system design"
+
+- notes on the design; this is not an actual listing of the virtual file system
+
+- `content` - `GitDir(".", "content")`
+
+    - `GitDir` is a special filesystem which makes all files have subpaths with commit data sourced from git.
+    their entries are ordered by how new/old a commit is
+
+        - `inner/<commit>` - contains the file content and a revision info fork
+
+        - `inner/latest` - same but for the latest revision, if applicable.
+        this may be the working tree
+
+- `template` - `PhysicalDir("template")`
+
+- `static` - `PhysicalDir("static")`