fix(tui): make /speak slash command dispatch immediately

[dgageot]

Problem

Typing /speak in the chat input did nothing — the text was silently forwarded to the agent as a regular message instead of starting speech-to-text transcription.

Root cause

commands.Parser.Parse only dispatches slash commands that have Immediate: true:

if item.SlashCommand == cmd && item.Immediate {
    return item.Execute(arg)
}

Every session slash command (/clear, /new, /exit, /yolo, …) sets Immediate: true, but speakCommand in pkg/tui/commands/speak_darwin.go was missing the flag, so /speak fell through to the regular message path.

Fix

• Set Immediate: true on the /speak command. Starting transcription is a local UI action that does not interrupt any ongoing stream, consistent with the other session slash commands.
• Add a darwin-only regression test asserting that parser.Parse("/speak") returns a command that emits StartSpeakMsg.

Validation

• go build ./...
• go test ./pkg/tui/commands/...
• mise lint