#23369 cmux-next tests: RemoteChildSessionsTests compiles again (FakeClient gets isReadOnly)

Summary

CmuxNextAppTests does not compile on feat-cmux-next at c2b90f6d9e04 (and still at 5c4e212b6bba):

RemoteChildSessionsTests.swift:14:37: error: type 'RemoteChildSessionsTests.FakeClient' does not conform to protocol 'AgentSessionRemoteClient'
AgentSessionRemoteClient.swift:9:9: note: protocol requires property 'isReadOnly' with type 'Bool'

#21258 added isReadOnly to AgentSessionRemoteClient. #23163 merged after it with a FakeClient that lacks the property. Each PR was green on its own base, but together they break the build. Every swift build --build-tests of Packages/macOS/CmuxNext now fails, so every PR's cmux-next swift test and every fleet suite step on CmuxNext fail before running a test.

The fix is one line: the fake wire says it is not read-only (let isReadOnly = false), which matches what a prompting wire is. RemoteAcpmuxObserverTests.FakeClient uses the same form.

Testing

• Fleet step at feat-cmux-next c2b90f6d9e04, package-test-lane.sh suite Packages/macOS/CmuxNext RemoteChildSessionsTests: build fails with the error above (step 2d8b61ef14262a94f0744411).
• Fleet step at this PR's head 6457a7b766ba: the build passes and all four tests pass (theChatsWatchListsItsChildrenAndTheirParentsAreNoted, linksKeepOneParentEachAndALoopEnds, aRemoteChildsWorkspaceNestsUnderItsParents, aRemoteChatOpensItsListedChildBesideIt), step 52226614e4a1779460e886a1.

No capture: test-only change

🤖 Generated with Claude Code
Agent session: cosmic-gray

---

^{Need help on this PR? Tag @codesmith-bot with what you need.}

---

Summary by cubic

Restores the CmuxNext test build on feat-cmux-next so RemoteChildSessionsTests compiles again.

#21258 added isReadOnly to AgentSessionRemoteClient; #23163 merged after it with a FakeClient that lacks the property, so every swift build --build-tests of Packages/macOS/CmuxNext failed. This adds let isReadOnly = false, matching the form RemoteAcpmuxObserverTests.FakeClient uses — the fake wire reports not read-only, which is what a prompting wire is.