CLI UX: Input prompt (»), G> output prefix, answering spinner

Three UX improvements for interactive mode:

1. Input prompt: Shows '»' (bold cyan) on stdin before reading user input
2. Output prefix: Agent responses prefixed with 'G>' (bold green)
3. Thinking indicator: Animated spinner on stderr showing 'answering (N.Ns)'
   with braille frame characters, auto-clears when response arrives
   - Start/stop via tokio::sync::watch channel + tokio::spawn
   - Only shows when interactive (atty detection)
   - Does not affect gateway/sub-agent modes

crates/gyandoot-agent/Cargo.toml

@@ -22,4 +22,5 @@ uuid.workspace = true
 chrono.workspace = true
 async-trait.workspace = true
 rand.workspace = true
-dirs.workspace = true
+dirs.workspace = true
+atty.workspace = true