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/server/server.go

@@ -78,7 +78,9 @@ func New(
 			srv.enableSentry()
 			srv.SetupRoutes()
 
-			go srv.serve() //nolint:contextcheck
+			// The start hook's context ends when the hook
+			// returns; serving must outlive it.
+			go srv.serve() //nolint:contextcheck,gosec // G118
 
 			return nil
 		},