fix: enrich runtime state before restore check, remove dead branchExists

- Add enrichSessionWithRuntimeState() call before isRestorable() in
  restore() so crashed sessions (status "working", agent exited) are
  correctly detected as terminal and eligible for restore.
- Remove dead branchExists from SCM interface and scm-github plugin
  (defined but never called anywhere in the codebase).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/core/src/session-manager.ts

@@ -903,8 +903,13 @@ export function createSessionManager(deps: SessionManagerDeps): SessionManager {
       throw new Error(`Session ${sessionId} not found`);
     }
 
-    // 2. Reconstruct Session from metadata
+    // 2. Reconstruct Session from metadata and enrich with live runtime state.
+    //    metadataToSession sets activity: null, so without enrichment a crashed
+    //    session (status "working", agent exited) would not be detected as terminal
+    //    and isRestorable would reject it.
     const session = metadataToSession(sessionId, raw);
+    const plugins = resolvePlugins(project);
+    await enrichSessionWithRuntimeState(session, plugins, true);
 
     // 3. Validate restorability
     if (!isRestorable(session)) {
@@ -914,8 +919,7 @@ export function createSessionManager(deps: SessionManagerDeps): SessionManager {
       throw new SessionNotRestorableError(sessionId, "session is not in a terminal state");
     }
 
-    // 4. Resolve plugins
-    const plugins = resolvePlugins(project);
+    // 4. Validate required plugins (plugins already resolved above for enrichment)
     if (!plugins.runtime) {
       throw new Error(`Runtime plugin '${project.runtime ?? config.defaults.runtime}' not found`);
     }

packages/core/src/types.ts

@@ -535,9 +535,6 @@ export interface SCM {
 
   /** Check if PR is ready to merge */
   getMergeability(pr: PRInfo): Promise<MergeReadiness>;
-
-  /** Optional: check if a branch exists in the repo */
-  branchExists?(repoPath: string, branch: string): Promise<boolean>;
 }
 
 // --- PR Types ---

packages/plugins/scm-github/src/index.ts

@@ -560,26 +560,6 @@ function createGitHubSCM(): SCM {
         blockers,
       };
     },
-    async branchExists(repoPath: string, branch: string): Promise<boolean> {
-      try {
-        await execFileAsync("git", ["rev-parse", "--verify", `refs/heads/${branch}`], {
-          cwd: repoPath,
-          timeout: 30_000,
-        });
-        return true;
-      } catch {
-        // Try remote ref
-        try {
-          await execFileAsync("git", ["rev-parse", "--verify", `refs/remotes/origin/${branch}`], {
-            cwd: repoPath,
-            timeout: 30_000,
-          });
-          return true;
-        } catch {
-          return false;
-        }
-      }
-    },
   };
 }