fix(auto): cache lastCommandCtx to survive shortcut-handler restarts

Shortcut handlers (registerShortcut) receive ExtensionContext which has
no newSession(). This caused autonomous mode started via Ctrl+Y to
always crash with 'newSession is not a function'.

- AutoSession.lastCommandCtx: new field that persists across stopAuto/reset
  so shortcut handlers can fall back to the last valid command context
- startAuto(): cache valid command ctx; fall back and notify user if ctx
  has no newSession; return early with actionable message if no cache yet
- dispatchHookUnit(): same guard — resolve hookCtx before s.cmdCtx = ctx
- run-unit.js: last-resort guard before newSession() call returns clean
  error category instead of TypeError
- steerable-autonomous-extension.js: rename ctrl+y → ctrl+alt+y to avoid
  conflict with terminal yank built-in

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/resources/extensions/sf/auto.js

@@ -1401,7 +1401,7 @@ export async function startAuto(ctx, pi, base, verboseMode, options) {
 	}
 	// ── Command context guard ────────────────────────────────────────────────
 	// Autonomous mode requires a ctx with newSession() to start clean sessions
-	// for each unit. Shortcut handlers (Ctrl+Y, registerShortcut) receive an
+	// for each unit. Shortcut handlers (Ctrl+Alt+Y, registerShortcut) receive an
 	// ExtensionContext which does NOT have newSession. Fall back to the last
 	// known command ctx if available; otherwise block with an actionable message.
 	if (typeof ctx.newSession !== "function") {

src/resources/extensions/sf/steerable-autonomous-extension.js

@@ -78,7 +78,7 @@ export default function steerableAutonomousExtension(api) {
 		},
 	});
 
-	api.registerShortcut("ctrl+y", {
+	api.registerShortcut("ctrl+alt+y", {
 		description: "Toggle YOLO mode (build + autonomous + deep + unrestricted; bypass git prompts). If not running, starts the autonomous loop immediately.",
 		handler: async (ctx) => {
 			const session = getAutoSession();