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/hashcash/hashcash.go

@@ -100,9 +100,10 @@ func (v *Validator) Validate(
 	dateStr := parts[2]
 	resource := parts[3]
 
-	if err := v.validateHeader(
+	err := v.validateHeader(
 		version, bitsStr, resource, requiredBits,
-	); err != nil {
+	)
+	if err != nil {
 		return err
 	}
 
@@ -111,13 +112,13 @@ func (v *Validator) Validate(
 		return err
 	}
 
-	if err := validateTime(stampTime); err != nil {
+	err = validateTime(stampTime)
+	if err != nil {
 		return err
 	}
 
-	if err := validateProof(
-		stamp, requiredBits,
-	); err != nil {
+	err = validateProof(stamp, requiredBits)
+	if err != nil {
 		return err
 	}