fix(call-omo-agent): track reused sync sessions

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/tools/call-omo-agent/sync-executor-leak.test.ts

@@ -45,6 +45,7 @@ function createDependencies(overrides?: Partial<ExecuteSyncDeps>): ExecuteSyncDe
     waitForCompletion: mock(async () => {}),
     processMessages: mock(async () => "agent response"),
     setSessionFallbackChain: mock(() => {}),
+    clearSessionFallbackChain: mock(() => {}),
     ...overrides,
   }
 }
@@ -133,7 +134,7 @@ describe("executeSync session cleanup", () => {
   })
 
   describe("#given executeSync reuses an existing session", () => {
-    test("#when execution completes successfully #then the reused session stays tracked in both Sets", async () => {
+    test("#when execution completes successfully #then the reused session is tracked in both Sets", async () => {
       // given
       const sessionID = "ses-reused"
       const args = { ...createArgs(), session_id: sessionID }
@@ -141,10 +142,15 @@ describe("executeSync session cleanup", () => {
       const promptAsync = mock(async () => ({ data: {} }))
       const deps = createDependencies({
         createOrGetSession: mock(async () => ({ sessionID, isNew: false })),
+        waitForCompletion: mock(async (createdSessionID: string) => {
+          expect(createdSessionID).toBe(sessionID)
+          expect(subagentSessions.has(sessionID)).toBe(true)
+          expect(syncSubagentSessions.has(sessionID)).toBe(true)
+        }),
       })
 
-      subagentSessions.add(sessionID)
-      syncSubagentSessions.add(sessionID)
+      expect(subagentSessions.has(sessionID)).toBe(false)
+      expect(syncSubagentSessions.has(sessionID)).toBe(false)
 
       // when
       const result = await executeSync(args, toolContext, createContext(promptAsync) as never, deps)
@@ -154,5 +160,25 @@ describe("executeSync session cleanup", () => {
       expect(subagentSessions.has(sessionID)).toBe(true)
       expect(syncSubagentSessions.has(sessionID)).toBe(true)
     })
+
+    test("#when execution applies a fallback chain #then it clears that chain in finally", async () => {
+      // given
+      const sessionID = "ses-reused-fallback"
+      const args = { ...createArgs(), session_id: sessionID }
+      const toolContext = createToolContext()
+      const promptAsync = mock(async () => ({ data: {} }))
+      const clearSessionFallbackChain = mock(() => {})
+      const deps = createDependencies({
+        createOrGetSession: mock(async () => ({ sessionID, isNew: false })),
+        clearSessionFallbackChain,
+      })
+      const fallbackChain = [{ providers: ["openai"], model: "gpt-5.4" }]
+
+      // when
+      await executeSync(args, toolContext, createContext(promptAsync) as never, deps, fallbackChain)
+
+      // then
+      expect(clearSessionFallbackChain).toHaveBeenCalledWith(sessionID)
+    })
   })
 })