handlePlanModeCommand had branches for entering and pausing but fell
through to #enterPlanMode() when planModePaused was true, so once a
session entered plan mode it cycled forever between plan and plan_paused.
/goal and other mode-gated commands were permanently blocked because
they refuse to run while planModeEnabled || planModePaused is true.
Add a paused branch that clears the paused flag, resets the reentry
marker (so the next /plan logs as a fresh entry, not a resume), and
appends a mode_change to "none". Tools, model, and plan state were
already restored by the prior #exitPlanMode({ paused: true }), so no
extra cleanup is needed.
Fixes#2510
can1357