resume picker: resume the selected session, not the branch's latest

The picker is session-oriented but the action only passed the branch to runResume, which re-derived the latest checkpoint on that branch and ignored the selected session. So two sessions on the same branch both resumed the latest, and an idle session with a stored branch but no committed checkpoint was selectable yet resumed unrelated work (or failed with "No Entire checkpoint found").

Fix:

• Carry each session's checkpoint ID through the picker. A row is selectable only when it has BOTH a branch and a committed checkpoint; otherwise it's shown disabled with the reason (no branch / no committed checkpoint).
• On selection, resume that specific checkpoint via a new resumeSessionOnBranch/resumeByCheckpointID path that switches to the branch and restores the chosen session, instead of searching branch history. Checkout logic is shared with runResume via switchToBranchForResume (no duplication).

Tests: two sessions on one branch each carry their own checkpoint; resuming a specific checkpoint restores that session and not the newer one; a branch-only session with no checkpoint is not selectable.