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/ircserver/conn.go

@@ -62,7 +62,6 @@ type Conn struct {
 
 	lastQueueID int64
 	closed      bool
-	cancel      context.CancelFunc
 }
 
 func newConn(
@@ -151,8 +150,10 @@ func resolveHost(ctx context.Context, addr string) string {
 }
 
 // serve is the main loop for a single IRC client connection.
+// Cancelling ctx when it returns stops the relay goroutine.
 func (c *Conn) serve(ctx context.Context) {
-	ctx, c.cancel = context.WithCancel(ctx)
+	ctx, cancel := context.WithCancel(ctx)
+	defer cancel()
 	defer c.cleanup(ctx)
 
 	scanner := bufio.NewScanner(c.conn)
@@ -481,7 +482,7 @@ func (c *Conn) deliverMOTD() {
 		"- %s Message of the Day -", c.serverSfx,
 	))
 
-	for _, line := range strings.Split(motd, "\n") {
+	for line := range strings.SplitSeq(motd, "\n") {
 		c.sendNumeric(irc.RplMotd, "- "+line)
 	}