Re-vendor the canonical files from sneak/prompts at dd4027b (closes #112)

The shared files are fetched from sneak/prompts dd4027b, with this
repository's own entries after the shared content in .gitignore,
.editorconfig and .dockerignore.

Lint and tests are Dockerfile phases (golangci-lint v2.14.0, Debian Go
1.24.13) that the build stage depends on, and the Makefile targets call
the script/ entrypoints. make fmt also formats Markdown with prettier.

Fixes for the new lint findings: the auth cookie is always Secure, an
IRC connection's relay goroutine stops when the connection closes, and
repeated strings are constants.

Model: opus-5-5

internal/handlers/handlers.go

@@ -24,6 +24,14 @@ import (
 
 var errUnauthorized = errors.New("unauthorized")
 
+// Field names and values used in many JSON responses.
+const (
+	statusKey   = "status"
+	statusError = "error"
+	errorKey    = "error"
+	nickKey     = "nick"
+)
+
 // Params defines the dependencies for creating Handlers.
 type Params struct {
 	fx.In
@@ -137,7 +145,7 @@ func (hdlr *Handlers) respondError(
 ) {
 	hdlr.respondJSON(
 		writer, request,
-		map[string]string{"error": msg},
+		map[string]string{errorKey: msg},
 		status,
 	)
 }