docs: record the branch/release model + sanitize progress

Document the main=dev / v1=release-pointer model in flake.nix (where the
v1 ref is defined) and mark the repo-sanitize roadmap items shipped
(installer prune, option-table reconcile, settings untrack, branch
realignment).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

flake.nix

@@ -51,8 +51,11 @@
           (nixpkgs.lib.concatStringsSep "\n"
             (builtins.attrNames nixos-hardware.nixosModules));
         nixpkgsPath = nixpkgs.outPath;
-        # v1 is the release branch: installed machines track it on
-        # `nix flake update` instead of the forge's default branch.
+        # Branch model: `main` is the development default; `v1` is the
+        # release pointer — the stable line for the NixOS 26.05 rewrite,
+        # advanced to main once a batch is tested (rolling, no tags). A
+        # future major bump would become `v2`. Installed machines track
+        # `?ref=v1` on `nix flake update`, not the forge's default branch.
         flakeUrl = "git+${gitUrl}?ref=v1";
         # Future updates re-resolve from the forge (original); the install
         # itself resolves from the ISO store (locked = path) — fully