feat: add irc numerics package, deduplicate constants, fix dead code

- Create internal/irc/ package with all IRC numeric reply codes (RFC 1459/2812)
  and command string constants as the single source of truth
- Replace all 69+ bare numeric string literals in api.go with named constants
  (e.g. irc.RplWelcome, irc.ErrNoSuchChannel)
- Add 'code' (int) and named 'command' (e.g. RPL_YOURHOST) fields to IRC
  message JSON replies via irc.Name() lookup in scanMessages
- Deduplicate command constants: remove local definitions from api.go,
  cmd/neoirc-cli/main.go, and cmd/neoirc-cli/api/client.go; all now import
  from internal/irc
- Fix dead code: remove handleListCmd/handleWhoCmd/handleWhoisCmd/
  sendWhoisNumerics that were unreachable due to dispatchCommand routing
  LIST/WHO/WHOIS to dispatchInfoCommand before dispatchQueryCommand.
  Route these commands to dispatchQueryCommand which has the improved
  implementations (e.g. ListAllChannelsWithCounts single-query vs N+1)
- Update enqueueNumeric and respondIRCError signatures from string to int
- Update test helper findNumeric to check the new 'code' JSON field

Closes #52

cmd/neoirc-cli/api/client.go

@@ -13,15 +13,14 @@ import (
 	"strconv"
 	"strings"
 	"time"
+
+	"git.eeqj.de/sneak/neoirc/internal/irc"
 )
 
 const (
 	httpTimeout      = 30 * time.Second
 	pollExtraTime    = 5
 	httpErrThreshold = 400
-
-	cmdJoin = "JOIN"
-	cmdPart = "PART"
 )
 
 var errHTTP = errors.New("HTTP error")
@@ -171,7 +170,7 @@ func (client *Client) PollMessages(
 func (client *Client) JoinChannel(channel string) error {
 	return client.SendMessage(
 		&Message{ //nolint:exhaustruct // only command+to needed
-			Command: cmdJoin, To: channel,
+			Command: irc.CmdJoin, To: channel,
 		},
 	)
 }
@@ -180,7 +179,7 @@ func (client *Client) JoinChannel(channel string) error {
 func (client *Client) PartChannel(channel string) error {
 	return client.SendMessage(
 		&Message{ //nolint:exhaustruct // only command+to needed
-			Command: cmdPart, To: channel,
+			Command: irc.CmdPart, To: channel,
 		},
 	)
 }