Say where a repository's own .gitignore and .editorconfig entries go (closes #103)

The canonical `.gitignore` had no place for a repository's own build outputs, and nothing said a repository's own `.editorconfig` sections survive a re-vendor, so re-vendoring dropped them: a Go repository lost `/bin/`, `*.test` and `*.out`, and its tabs for Go files.

`.gitignore` now ends with a section, like `.dockerignore`'s header, where a repository adds its own build outputs (a root binary written `/myapp`) and its own environment-template negations, which a re-vendor keeps. `.editorconfig` gains `[*.go]` with tabs and the same kind of closing note. `prompts/REPO_POLICIES.md`, both checklists and the Go styleguide say these two files are the canonical content followed by the repository's own entries. Closes #104 too.

Model: opus-5-5

prompts/CODE_STYLEGUIDE_GO.md

@@ -1,6 +1,6 @@
 ---
 title: Code Styleguide — Go
-last_modified: 2026-10-04
+last_modified: 2026-10-06
 ---
 
 1.  Try to hard wrap long lines at 77 characters or less.
@@ -148,8 +148,9 @@ last_modified: 2026-10-04
     handle HTTP requests. Don't use methods or your top level functions as
     handlers.
 
-1.  Provide a .gitignore file that ignores at least `*.log`, `*.out`, and
-    `*.test` files, as well as any binaries.
+1.  The repository's own entries at the end of `.gitignore`, which a re-vendor
+    keeps, ignore at least `*.log`, `*.out`, and `*.test` files, as well as any
+    binaries.
 
 1.  Constructors **must** be called `New()`. `modulename.New()` works great if
     you name the packages properly. If the constructor creates an instance from