Replace TUI (ratatui) with REPL (rustyline + termimad)

Drop the full Ratatui TUI in favor of a lighter REPL channel built on
rustyline (line editing, history, tab-completion) and termimad (inline
markdown rendering). Removes ratatui and crossterm event-stream deps,
adds rustyline and termimad. Simplifies main.rs startup to use the REPL
directly instead of the alternate-screen TUI.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

src/cli/mod.rs

@@ -41,10 +41,6 @@ pub struct Cli {
     #[arg(long, global = true)]
     pub no_db: bool,
 
-    /// Simple REPL mode without TUI (for testing)
-    #[arg(long, global = true)]
-    pub repl: bool,
-
     /// Single message mode - send one message and exit
     #[arg(short, long, global = true)]
     pub message: Option<String>,