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>

README.md

@@ -21,7 +21,7 @@ Working:
 - VAAPI H.264 encode in GStreamer (RDNA3 confirmed; other VAAPI-capable
   GPUs should work), with a software x264 fallback via `--no-hwencode`
 - Audio capture of the default sink's monitor, with optional per-app
-  routing (`--app <name>`) and microphone mixing (`--mic`)
+  routing (`--app <name>`)
 - `--repair` cleanup of orphaned PipeWire state left by a crashed host
 - iroh QUIC bi-stream tunnel, direct-UDP and relay paths both verified
 - Interactive Host/View menu with clipboard auto-copy and mpv/VLC picker
@@ -190,16 +190,16 @@ measured_at = "2026-05-21T20:41:16Z"
 ## Audio
 
 By default pixelpass captures the default sink's monitor — the viewer
-hears whatever the host hears. Two flags adjust this:
+hears whatever the host hears. `--app <name>` narrows that to a single
+application: pixelpass creates a per-PID null-sink and uses libpipewire to
+reroute matching `Stream/Output/Audio` nodes (by `application.name`) into
+it, so the viewer hears just that app instead of the whole desktop. In the
+interactive menu you can pick the app from a list of what's currently
+playing.
 
-- `--app <name>` routes only a single application's audio. pixelpass
-  creates a per-PID null-sink and uses libpipewire to reroute matching
-  `Stream/Output/Audio` nodes (by `application.name`) into it, so the
-  viewer hears just that app instead of the whole desktop. In the
-  interactive menu you can pick the app from a list of what's currently
-  playing.
-- `--mic` mixes the default microphone source into the stream alongside
-  system audio.
+Microphone capture is intentionally 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.
 
 If a host crashes mid-session it can leave orphaned `pixelpass_capture_*`
 null-sinks and their paired loopbacks loaded in PipeWire. Run