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

.editorconfig

@@ -10,3 +10,9 @@ insert_final_newline = true
 
 [Makefile]
 indent_style = tab
+
+[*.go]
+indent_style = tab
+
+# This repository's own sections, such as one for another language it
+# uses, go below this comment, and a re-vendor keeps them.