fix(session): preserve custom agent after switching (#1017)

Use setSessionAgent (first-write wins) instead of updateSessionAgent in chat.message handler. This prevents the default agent from overwriting a custom agent that was set via UI switch.

Fixes #893

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

Co-authored-by: justsisyphus <justsisyphus@users.noreply.github.com>
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/features/claude-code-session-state/state.test.ts

@@ -123,4 +123,40 @@ describe("claude-code-session-state", () => {
       expect(getSessionAgent(sessionID)).toBeUndefined()
     })
   })
+
+  describe("issue #893: custom agent switch reset", () => {
+    test("should preserve custom agent when default agent is sent on subsequent messages", () => {
+      // #given - user switches to custom agent "MyCustomAgent"
+      const sessionID = "test-session-custom"
+      const customAgent = "MyCustomAgent"
+      const defaultAgent = "Sisyphus"
+
+      // User switches to custom agent (via UI)
+      setSessionAgent(sessionID, customAgent)
+      expect(getSessionAgent(sessionID)).toBe(customAgent)
+
+      // #when - first message after switch sends default agent
+      // This simulates the bug: input.agent = "Sisyphus" on first message
+      // Using setSessionAgent (first-write wins) should preserve custom agent
+      setSessionAgent(sessionID, defaultAgent)
+
+      // #then - custom agent should be preserved, NOT overwritten
+      expect(getSessionAgent(sessionID)).toBe(customAgent)
+    })
+
+    test("should allow explicit agent update via updateSessionAgent", () => {
+      // #given - custom agent is set
+      const sessionID = "test-session-explicit"
+      const customAgent = "MyCustomAgent"
+      const newAgent = "AnotherAgent"
+
+      setSessionAgent(sessionID, customAgent)
+
+      // #when - explicit update (user intentionally switches)
+      updateSessionAgent(sessionID, newAgent)
+
+      // #then - should be updated
+      expect(getSessionAgent(sessionID)).toBe(newAgent)
+    })
+  })
 })