feat: implement IRC numerics batch 2 — connection registration, channel ops, user queries (#59)

## Summary

Implements the remaining important/commonly-used IRC numeric reply codes, as requested in [issue #52](#52).

### Connection Registration (001-005)
- **002 RPL_YOURHOST** — "Your host is <server>, running version <ver>"
- **003 RPL_CREATED** — "This server was created <date>"
- **004 RPL_MYINFO** — "<server> <version> <usermodes> <chanmodes>"
- **005 RPL_ISUPPORT** — CHANTYPES=#, NICKLEN=32, CHANMODES, NETWORK=neoirc, CASEMAPPING=ascii

All sent automatically after RPL_WELCOME during session creation/login.

### Server Statistics (251-255)
- **251 RPL_LUSERCLIENT** — user count
- **252 RPL_LUSEROP** — operator count
- **254 RPL_LUSERCHANNELS** — channel count
- **255 RPL_LUSERME** — local client count

Sent during connection registration and available via LUSERS command.

### Channel Operations
- **MODE command** — query channel modes (324 RPL_CHANNELMODEIS + 329 RPL_CREATIONTIME) and user modes (221 RPL_UMODEIS)
- **NAMES command** — query channel member list (reuses 353/366)
- **LIST command** — list all channels with member counts (322 RPL_LIST + 323 end)

### User Queries
- **WHOIS command** — 311 RPL_WHOISUSER, 312 RPL_WHOISSERVER, 319 RPL_WHOISCHANNELS, 318 RPL_ENDOFWHOIS
- **WHO command** — 352 RPL_WHOREPLY, 315 RPL_ENDOFWHO

### Database Additions
- `GetChannelCount()` — total channel count for LUSERS
- `ListAllChannelsWithCounts()` — channels with member counts for LIST
- `GetChannelCreatedAt()` — channel creation time for RPL_CREATIONTIME
- `GetSessionCreatedAt()` — session creation time

### Other Changes
- Added `StartTime` to `Globals` struct for RPL_CREATED
- Updated README with comprehensive documentation of all new commands and numerics
- Updated roadmap to reflect implemented features

`docker build .` passes (lint, tests, build all green).

closes [#52](#52)

<!-- session: agent:sdlc-manager:subagent:1f3dcab8-ad6a-4c4c-af72-34a617640c9d -->

Co-authored-by: clawbot <clawbot@noreply.git.eeqj.de>
Co-authored-by: clawbot <clawbot@git.eeqj.de>
Reviewed-on: #59
Co-authored-by: clawbot <sneak+clawbot@sneak.cloud>
Co-committed-by: clawbot <sneak+clawbot@sneak.cloud>

cmd/neoirc-cli/api/client.go

@@ -13,6 +13,8 @@ import (
 	"strconv"
 	"strings"
 	"time"
+
+	"git.eeqj.de/sneak/neoirc/internal/irc"
 )
 
 const (
@@ -168,7 +170,7 @@ func (client *Client) PollMessages(
 func (client *Client) JoinChannel(channel string) error {
 	return client.SendMessage(
 		&Message{ //nolint:exhaustruct // only command+to needed
-			Command: "JOIN", To: channel,
+			Command: irc.CmdJoin, To: channel,
 		},
 	)
 }
@@ -177,7 +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: "PART", To: channel,
+			Command: irc.CmdPart, To: channel,
 		},
 	)
 }