feat(server): cache getPullRequestStatus with TTL and in-flight dedup

Adds @isaacs/ttlcache to avoid repeated gh CLI calls for the same
working directory. Concurrent lookups for the same cwd share a single
in-flight promise. Cache expires after 30s by default.

package-lock.json

@@ -35318,6 +35318,7 @@
         "@deepgram/sdk": "^3.4.0",
         "@getpaseo/highlight": "0.1.37",
         "@getpaseo/relay": "0.1.37",
+        "@isaacs/ttlcache": "^2.1.4",
         "@modelcontextprotocol/sdk": "^1.20.1",
         "@opencode-ai/sdk": "1.2.6",
         "@sctg/sentencepiece-js": "^1.1.0",
@@ -35360,6 +35361,15 @@
         "vitest": "^3.2.4"
       }
     },
+    "packages/server/node_modules/@isaacs/ttlcache": {
+      "version": "2.1.4",
+      "resolved": "https://registry.npmjs.org/@isaacs/ttlcache/-/ttlcache-2.1.4.tgz",
+      "integrity": "sha512-7kMz0BJpMvgAMkyglums7B2vtrn5g0a0am77JY0GjkZZNetOBCFn7AG7gKCwT0QPiXyxW7YIQSgtARknUEOcxQ==",
+      "license": "BlueOak-1.0.0",
+      "engines": {
+        "node": ">=12"
+      }
+    },
     "packages/server/node_modules/@modelcontextprotocol/sdk": {
       "version": "1.20.1",
       "resolved": "https://registry.npmjs.org/@modelcontextprotocol/sdk/-/sdk-1.20.1.tgz",