add-channel-typelisted

Scaffold a new channel type under `internal/channel/`. Ask the user for: 1. **Channel type name** — lowercase (e.g. `slack`, `discord`, `whatsapp`) 2. **Markup format** — `MarkupMarkdown` or `MarkupHTML` 3. **Needs provisioner?** — for ephemeral channel auto-setup (e.g. creating bots) 4. **Needs platform state?** — for persistent platform-specific data (e.g. group IDs) 5. **Config fields** — what goes in tclaw.yaml for this channel type (e.g. token, webhook URL) ## Reference patterns Before writing any code, read these files to match the exact patterns: - `internal/channel/socketchannel/package.go` — minimal package (no provisioner) - `internal/channel/telegramchannel/package.go` — package with provisioner - `internal/channel/channelpkg/channelpkg.go` — the `Package` interface and `BuildParams` - `internal/channel/channel.go` — `Channel` interface, `ChannelType` constants, `Markup` types - `internal/channel/platform_state.go` — platform state types - `internal/channel/all/all.go` — where packages are registered - `internal/config/config.go` — config structs for channel types Also read `docs/go-patterns.md` for code style rules. ## Files to create/modify For a channel type named `{name}`: ### `internal/channel/{name}channel/{name}.go` - Transport struct implementing `channel.Channel` interface - Methods: `Send()`, `Receive()`, `Close()`, `Markup()`, `StatusWrap()`, `SplitStatusMessages()`, `FormattingInstructions()` ### `internal/channel/{name}channel/package.go` - St