Match .gitignore, .editorconfig and .golangci.yml to sneak/prompts (closes #174)

REPO_POLICIES.md asks for the standard .gitignore and .editorconfig
followed by the repository's own entries, and for the canonical
.golangci.yml. The shared parts and .golangci.yml now match the
sneak/prompts copies at 065357920a0a09bd736bf33d4cb5ce935fdc2328 byte
for byte. .editorconfig has no entries of its own left, since the
standard file now carries the [*.go] section. .golangci.yml disables
canonicalheader, as the canonical copy does.

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]
@@ -46,7 +46,9 @@ node_modules/
 [iI][dD]_[eE][dD]25519
 [iI][dD]_[eE][dD]25519_[sS][kK]
 
-# This repository's own entries, after the shared content above.
+# 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/`.
 
 # Build output
 dist/