Keep PR status current after agent merges (#1455)

* feat(server): refresh PR state promptly after agent merges via gh CLI

When an agent merges a PR with gh pr merge, the app could take up to
2 minutes to show the merged state because:

- gh pr merge is a remote GitHub operation that does not touch local git refs,
  so file watchers never fire
- the GitHub self-heal poll runs on a 2-minute slow interval for settled PRs
- no refresh is triggered when agent tool calls complete

Add a detection path that intercepts completed shell tool calls in the
agent manager and signals the workspace git service when a command may
have changed external state (gh *, git push, npm publish). The git
service then invalidates the GitHub cache and schedules a forced
GitHub-inclusive refresh, so the updated PR state arrives within ~500ms.

The agent manager does not leak cache semantics; it calls a semantic
onWorkspaceStateMayHaveChanged callback and lets the git service decide
what that means internally.

* refactor(server): share workspace refresh merge logic

packages/server/src/server/session.workspaces.test.ts

@@ -521,6 +521,7 @@ function createSessionForWorkspaceTests(
           unsubscribe: () => {},
         }),
         scheduleRefreshForCwd: () => {},
+        onWorkspaceStateMayHaveChanged: () => {},
         getMetrics: () => ({
           checkoutDiffTargetCount: 0,
           checkoutDiffSubscriptionCount: 0,
@@ -631,6 +632,7 @@ test("create_agent_request keeps requested child cwd when grouped under an exist
             unsubscribe: () => {},
           }),
           scheduleRefreshForCwd: () => {},
+          onWorkspaceStateMayHaveChanged: () => {},
           getMetrics: () => ({
             checkoutDiffTargetCount: 0,
             checkoutDiffSubscriptionCount: 0,
@@ -1013,6 +1015,7 @@ test("archive emits an authoritative agent_update upsert for subscribed clients"
           unsubscribe: () => {},
         }),
         scheduleRefreshForCwd: () => {},
+        onWorkspaceStateMayHaveChanged: () => {},
         getMetrics: () => ({
           checkoutDiffTargetCount: 0,
           checkoutDiffSubscriptionCount: 0,
@@ -1373,6 +1376,7 @@ test("close_items_request archives agents and kills terminals in one batch", asy
           unsubscribe: () => {},
         }),
         scheduleRefreshForCwd: () => {},
+        onWorkspaceStateMayHaveChanged: () => {},
         getMetrics: () => ({
           checkoutDiffTargetCount: 0,
           checkoutDiffSubscriptionCount: 0,
@@ -1562,6 +1566,7 @@ test("close_items_request archives stored agents that are not currently loaded",
           unsubscribe: () => {},
         }),
         scheduleRefreshForCwd: () => {},
+        onWorkspaceStateMayHaveChanged: () => {},
         getMetrics: () => ({
           checkoutDiffTargetCount: 0,
           checkoutDiffSubscriptionCount: 0,
@@ -1712,6 +1717,7 @@ test("close_items_request continues after an archive failure", async () => {
           unsubscribe: () => {},
         }),
         scheduleRefreshForCwd: () => {},
+        onWorkspaceStateMayHaveChanged: () => {},
         getMetrics: () => ({
           checkoutDiffTargetCount: 0,
           checkoutDiffSubscriptionCount: 0,
@@ -2578,6 +2584,7 @@ test("workspace update stream keeps persisted workspace visible after agents sto
           unsubscribe: () => {},
         }),
         scheduleRefreshForCwd: () => {},
+        onWorkspaceStateMayHaveChanged: () => {},
         getMetrics: () => ({
           checkoutDiffTargetCount: 0,
           checkoutDiffSubscriptionCount: 0,