Add F12 hotkey to toggle microphone AGC

Automatic gain control was always on with no way to turn it off. Toggle
it with F12, the /agc command, or the agc FIFO command, and persist the
choice in the configuration file the same way noise suppression does.

AgcEnabled defaults to true so existing setups keep their current
behavior, and the saved value is applied to the stream on connect. The
enabled flag becomes an atomic.Bool because the capture goroutine reads
it while the UI goroutine writes it, and the lazily created right
channel AGC now inherits the left channel's state.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -40,6 +40,20 @@ noisesuppressionenabled = true
 
 RNNoise is a required build and runtime dependency.
 
+## Automatic Gain Control
+
+Barnard normalizes the level of your outgoing microphone audio with automatic gain control (AGC), which boosts quiet speech and compresses loud peaks. AGC is enabled by default.
+
+### Controls
+- **F12 key**: Toggle AGC on/off (configurable hotkey)
+- **FIFO command**: Send `agc` command to toggle during runtime
+- **Configuration**: Set `agcenabled` in `~/.barnard.toml`
+
+### Configuration Example
+```toml
+agcenabled = true
+```
+
 ## FIFO Control
 
 If you pass the --fifo option to Barnard, a FIFO pipe will be created.
@@ -54,6 +68,7 @@ Current Commands:
 * toggle: Toggle your transmission state.
 * talk: Synonym for toggle.
 * noise: Toggle noise suppression on/off for microphone input.
+* agc: Toggle automatic gain control on/off for microphone input.
 * record: Toggle recording. You may also use `record start` or `record stop`.
 * exit: Exit Barnard, just like when you press your quit key.
 
@@ -278,6 +293,7 @@ After running the command above, `barnard` will be compiled as `$(go env GOPATH)
 
 - <kbd>F1</kbd>: toggle voice transmission
 - <kbd>F9</kbd>: toggle noise suppression
+- <kbd>F12</kbd>: toggle automatic gain control
 - <kbd>F11</kbd>: open actions menu for the focused tree item
 - <kbd>Ctrl+R</kbd>: toggle recording
 - <kbd>Ctrl+L</kbd>: clear chat log