Extract the duplicated commandContextActions implementations from
rpc-mode.ts and print-mode.ts into a shared
createDefaultCommandContextActions() factory in
modes/shared/command-context-actions.ts.
Both headless modes (RPC and print) had identical implementations of
waitForIdle, newSession, fork, navigateTree, switchSession, and reload
that simply delegate to AgentSession. The shared factory provides these
defaults; interactive mode continues to layer TUI-specific behavior on
top via its own overrides.
Also fixes a subtle redundancy in print mode where newSession manually
called options.setup after session.newSession(), even though
session.newSession() already handles the setup callback internally.
