Hot-swap effort level on Claude session via applyFlagSettings (#314295)

Use the SDK's applyFlagSettings({ effortLevel }) to update reasoning
effort on an active Claude session, matching the model and permission-mode
hot-swap pattern. This avoids the previous workaround of restarting the
session whenever the user changed effort level.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>