Plan mode: no way to switch plans or re-enter planning after exit

Problem

Once ExitPlanMode is called and execution starts, there's no user-accessible way to:

• Re-enter plan mode to revise the approved plan if the work reveals the plan was wrong.
• Switch to a different plan (e.g. load a plan saved previously under ~/.claude/plans/ into the current session's plan slot).
• View an older plan file in the plan UI — Read-ing the .md file just dumps its contents to the transcript.

The practical consequence: plans are single-use and tied to the moment of approval. If the user has a library of plan files from past sessions, there's no way to pick one up again with the same UI affordances.

Suggestion

• A way to re-enter plan mode mid-session (slash command or UI action) that writes the current plan file's contents back as the editable draft.
• A plan picker that can load any .md under the plans directory into the plan UI for viewing/re-approval.
• Optional: allow the model to propose plan revisions during execution, which would surface through the same UI.

Environment

Claude Code, current version.