feat(nostr): nostr-based account registration and verification

Rebuilt the nostr registration feature cleanly on top of current ergo
mainline (origin/master), dropping the stale ergo reversions that were
entangled in the original squashed branch.

- New irc/nostr package: NIP-04/NIP-17 DMs, NIP-05 resolution, relay
  connect/auth, and identifier helpers.
- Registration/verification via nostr: parseCallback auto-detects nostr
  identifiers (NIP-05, npub, hex pubkey) and dispatches a verification DM.
- Nostr-based cloaked hostnames for accounts (opt-in via
  ip-cloaking.nostr-hostnames), applied at the account-cloak sites.
- Config: accounts.registration.nostr-verification and the
  nostr-hostnames cloak option, with matching default.yaml and help text.
- Adds github.com/nbd-wtf/go-nostr and vendored dependencies.

vendor/github.com/coder/websocket/internal/bpool/bpool.go

@@ -0,0 +1,24 @@
+package bpool
+
+import (
+	"bytes"
+	"sync"
+)
+
+var bpool sync.Pool
+
+// Get returns a buffer from the pool or creates a new one if
+// the pool is empty.
+func Get() *bytes.Buffer {
+	b := bpool.Get()
+	if b == nil {
+		return &bytes.Buffer{}
+	}
+	return b.(*bytes.Buffer)
+}
+
+// Put returns a buffer into the pool.
+func Put(b *bytes.Buffer) {
+	b.Reset()
+	bpool.Put(b)
+}