feat: add traditional IRC wire protocol listener (closes #89) (#94)

## Summary

Adds a backward-compatible IRC wire protocol listener (RFC 1459/2812) that allows standard IRC clients (irssi, weechat, hexchat, etc.) to connect directly via TCP.

## Changes

### New package: `internal/ircserver/`

- **`parser.go`** — IRC wire protocol message parser and formatter
- **`server.go`** — TCP listener with Fx lifecycle integration
- **`conn.go`** — Per-connection handler with registration flow, PING/PONG, welcome burst
- **`commands.go`** — All IRC command handlers (JOIN, PART, PRIVMSG, MODE, TOPIC, KICK, WHOIS, etc.)
- **`relay.go`** — Message relay goroutine that delivers queued messages to IRC clients in wire format

### Modified files

- **`internal/config/config.go`** — Added `IRC_LISTEN_ADDR` environment variable
- **`internal/handlers/handlers.go`** — Broker is now injected via Fx (shared with IRC server)
- **`cmd/neoircd/main.go`** — Registered `broker.New`, `ircserver.New` as Fx providers
- **`pkg/irc/commands.go`** — Added `CmdUser` and `CmdInvite` constants
- **`README.md`** — Added IRC Protocol Listener documentation section

### Tests

- Parser unit tests (table-driven, round-trip verification)
- Integration tests: registration, PING/PONG, JOIN, PART, PRIVMSG (channel + DM), NICK change, duplicate nick rejection, LIST, WHOIS, QUIT, TOPIC, MODE, WHO, LUSERS, MOTD, AWAY, PASS, CAP negotiation, unknown commands, pre-registration errors
- Benchmarks for parser and formatter

## Key Design Decisions

- **Optional**: Listener is only started when `IRC_LISTEN_ADDR` is set
- **Shared infrastructure**: Same DB, broker, and session system as HTTP API
- **Full bridge**: IRC ↔ HTTP messages are interoperable
- **No schema changes**: Reuses existing tables
- **Broker as Fx dependency**: Extracted from handlers to be shared

## Supported Commands

Connection: NICK, USER, PASS, QUIT, PING/PONG, CAP
Channels: JOIN, PART, MODE, TOPIC, NAMES, LIST, KICK, INVITE
Messaging: PRIVMSG, NOTICE
Info: WHO, WHOIS, LUSERS, MOTD, AWAY, USERHOST
Operator: OPER

closes #89

Co-authored-by: user <user@Mac.lan guest wan>
Co-authored-by: Jeffrey Paul <sneak@noreply.example.org>
Reviewed-on: sneak/chat#94
Co-authored-by: clawbot <sneak+clawbot@sneak.cloud>
Co-committed-by: clawbot <sneak+clawbot@sneak.cloud>

internal/db/queries.go

@@ -2165,6 +2165,52 @@ func (database *Database) SetChannelSecret(
 	return nil
 }
 
+// --- No External Messages (+n) ---
+
+// IsChannelNoExternal checks if a channel has +n mode.
+func (database *Database) IsChannelNoExternal(
+	ctx context.Context,
+	channelID int64,
+) (bool, error) {
+	var isNoExternal int
+
+	err := database.conn.QueryRowContext(ctx,
+		`SELECT is_no_external FROM channels
+		 WHERE id = ?`,
+		channelID,
+	).Scan(&isNoExternal)
+	if err != nil {
+		return false, fmt.Errorf(
+			"check no external: %w", err,
+		)
+	}
+
+	return isNoExternal != 0, nil
+}
+
+// SetChannelNoExternal sets or unsets +n mode.
+func (database *Database) SetChannelNoExternal(
+	ctx context.Context,
+	channelID int64,
+	noExternal bool,
+) error {
+	val := 0
+	if noExternal {
+		val = 1
+	}
+
+	_, err := database.conn.ExecContext(ctx,
+		`UPDATE channels
+		 SET is_no_external = ?, updated_at = ?
+		 WHERE id = ?`,
+		val, time.Now(), channelID)
+	if err != nil {
+		return fmt.Errorf("set no external: %w", err)
+	}
+
+	return nil
+}
+
 // ListAllChannelsWithCountsFiltered returns all channels
 // with member counts, excluding secret channels that
 // the given session is not a member of.

internal/db/schema/001_initial.sql

@@ -44,6 +44,7 @@ CREATE TABLE IF NOT EXISTS channels (
     is_topic_locked INTEGER NOT NULL DEFAULT 1,
     is_invite_only INTEGER NOT NULL DEFAULT 0,
     is_secret INTEGER NOT NULL DEFAULT 0,
+    is_no_external INTEGER NOT NULL DEFAULT 1,
     channel_key TEXT NOT NULL DEFAULT '',
     user_limit INTEGER NOT NULL DEFAULT 0,
     created_at DATETIME DEFAULT CURRENT_TIMESTAMP,

internal/db/testing.go

@@ -0,0 +1,25 @@
+package db
+
+import (
+	"context"
+	"database/sql"
+	"log/slog"
+)
+
+// NewTestDatabaseFromConn creates a Database wrapping an
+// existing *sql.DB connection.  Intended for integration
+// tests in other packages.
+func NewTestDatabaseFromConn(conn *sql.DB) *Database {
+	return &Database{ //nolint:exhaustruct
+		conn: conn,
+		log:  slog.Default(),
+	}
+}
+
+// RunMigrations applies all schema migrations.  Exposed
+// for integration tests in other packages.
+func (database *Database) RunMigrations(
+	ctx context.Context,
+) error {
+	return database.runMigrations(ctx)
+}