feat: wire which-key popup into TUI with persistent display until resolution

Popup renders after prefix key (SPC, g) and stays visible indefinitely
until sequence completes, Escape cancels, or invalid key clears state.
No timeout — matches Doom Emacs behavior where which-key waits for user.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/claude-agent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

Cargo.lock

@@ -1660,6 +1660,7 @@ dependencies = [
  "tokio",
  "tonic",
  "tonic-build",
+ "which-key",
 ]
 
 [[package]]
@@ -1797,6 +1798,16 @@ dependencies = [
  "semver",
 ]
 
+[[package]]
+name = "which-key"
+version = "0.1.0"
+dependencies = [
+ "insta",
+ "proptest",
+ "ratatui",
+ "unicode-width 0.2.0",
+]
+
 [[package]]
 name = "winapi"
 version = "0.3.9"