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

The canonical `.gitignore` and `.editorconfig` now each end with a comment saying a repository's own entries go below it and a re-vendor keeps them, as `.dockerignore`'s header already does; without it a re-vendor dropped a repository's binaries and test outputs from `.gitignore`. `.editorconfig` gains `[*.go]` with tabs, since `gofmt` decides Go indentation in every repository. `prompts/REPO_POLICIES.md`, both checklists and the Go styleguide say the same in plain sentences. Common outputs such as `*.test` stay out of the canonical `.gitignore`. This also covers #104.

Model: opus-5-5

.gitignore

@@ -27,7 +27,7 @@ node_modules/
 # Environment files. `*.env` covers bare `.env` and the `prod.env`
 # convention. Only the templates `example.env` and `sample.env` are
 # re-included below. A repository that commits any other template adds
-# its own negation after these lines, for example `!.env.example`.
+# its own negation at the end of this file, for example `!.env.example`.
 *.[eE][nN][vV]
 .[eE][nN][vV].*
 .[eE][nN][vV][rR][cC]
@@ -45,3 +45,7 @@ node_modules/
 [iI][dD]_[eE][cC][dD][sS][aA]_[sS][kK]
 [iI][dD]_[eE][dD]25519
 [iI][dD]_[eE][dD]25519_[sS][kK]
+
+# This repository's own entries, such as its build outputs, go below
+# this comment, and a re-vendor keeps them. Anchor a binary built at the
+# root: `/myapp`, never `myapp`, which also ignores `cmd/myapp/`.