feat(cli): remove --mic (microphone is out of scope)

pixelpass is a screen-share tool meant to be paired with a dedicated
voice app (Mumble, TeamSpeak, Discord, …) for two-way talk — it never
mixes a mic. The --mic flag was declared, shown in the host banner, and
documented as working, but was never wired into the gst pipeline (a
no-op). Removed the flag from Cli + HostOpts + into_host_opts, dropped
it from the banner capture summary, and replaced the README's "--mic
mixes the mic" claim with an explicit out-of-scope note.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/host/mod.rs

@@ -342,8 +342,5 @@ fn capture_summary(opts: &HostOpts) -> String {
     } else {
         bits.push("system-audio".to_string());
     }
-    if opts.mic {
-        bits.push("mic".to_string());
-    }
     bits.join(" + ")
 }