feat(coding-agent/modes): implemented space-bar hold push-to-talk STT gesture

- Added a space-hold gesture state machine in CustomEditor, tracking repeated spaces, detecting holds beyond SPACE_HOLD_THRESHOLD, and firing start/end callbacks via a release timer.
- Hooked editor space-hold callbacks in InputController so STT toggles on hold start and again on release when STT is enabled.
- Added tests for space-hold start/stop behavior and updated keybinding docs to describe the hold-to-record STT workflow.

packages/coding-agent/src/modes/utils/hotkeys-markdown.ts

@@ -49,7 +49,7 @@ export function buildHotkeysMarkdown(bindings: HotkeysMarkdownBindings): string
 		`| \`${appKey(bindings, "app.thinking.toggle")}\` | Toggle thinking block visibility |`,
 		`| \`${appKey(bindings, "app.editor.external")}\` | Edit message in external editor |`,
 		`| \`${appKey(bindings, "app.clipboard.pasteImage")}\` | Paste image or text from clipboard |`,
-		`| \`${appKey(bindings, "app.stt.toggle")}\` | Toggle speech-to-text recording |`,
+		"| Hold `Space` | Speech-to-text (push-to-talk): hold to record, release to transcribe |",
 		`| \`${appKey(bindings, "app.agents.hub")}\` / \`${appKey(bindings, "app.session.observe")}\` / double-tap \`←\` (empty editor) | Open the agent hub |`,
 		"| `#` | Open prompt actions |",
 		"| `/` | Slash commands |",