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/EXISTING_REPO_CHECKLIST.md

@@ -1,6 +1,6 @@
 ---
 title: Existing Repo Checklist
-last_modified: 2026-10-04
+last_modified: 2026-10-06
 ---
 
 Use this checklist when beginning work in a repo that may not yet conform to our
@@ -28,13 +28,18 @@ with your task.
       root — never a file or directory named after one agent tool, such as
       `CLAUDE.md` or `.claude/`, and never separate memory files. Move what any
       such committed file says into `AGENTS.md` and delete it.
-- [ ] `.gitignore` is comprehensive (OS, editor, agent scratch, language
-      artifacts, secrets) — fetch from
+- [ ] `.gitignore` is comprehensive (OS, editor, agent scratch, secrets, the
+      repo's own build outputs) — fetch from
       `https://git.eeqj.de/sneak/prompts/raw/branch/main/.gitignore` if missing.
       An existing repo usually has a hand-written one that is never re-fetched,
-      so check the entries rather than the file's presence.
+      so check the entries rather than the file's presence. The file is the
+      canonical content followed by the repo's own entries, such as its
+      binaries; a re-vendor replaces the canonical part and keeps those entries.
 - [ ] `.editorconfig` exists — fetch from
-      `https://git.eeqj.de/sneak/prompts/raw/branch/main/.editorconfig`
+      `https://git.eeqj.de/sneak/prompts/raw/branch/main/.editorconfig`. The
+      file is the canonical content followed by the repo's own sections, such as
+      one for another language it uses; a re-vendor replaces the canonical part
+      and keeps those sections.
 - [ ] `Dockerfile` and `.dockerignore` exist; the Dockerfile carries a `lint`
       phase and a `test` phase, and the final stage carries a `COPY --from=` of
       a harmless file from each — fetch `.dockerignore` from