Plan mode: add "approve but defer execution" option for low-context sessions

Problem

When a plan is completed near the end of a session (context window running low), there's no clean way to approve the plan without triggering immediate execution. The current flow:

1. Plan is complete, Claude calls ExitPlanMode
2. User wants to approve but not execute (context too low for quality execution)
3. User rejects ExitPlanMode and explains "approve but execute next session"
4. Claude tries to do session wrap-up work (updating docs, committing), but plan mode keeps reasserting — blocking non-plan-file edits
5. Eventually the user accepts ExitPlanMode and Claude starts executing the plan, which is exactly what they didn't want

The plan file is already persisted to ~/.claude/plans/ and survives across sessions. The infrastructure for deferred execution exists — the UX just doesn't support it.

Expected behavior

ExitPlanMode should offer an additional option like "Approve plan for later execution" that:

• Saves the plan (already happens)
• Marks it as approved
• Exits plan mode back to normal conversation (so the user can do wrap-up: commit, update docs, etc.)
• Does not start implementation

Next session, Claude reads the approved plan from the plan file and can resume execution.

Current workaround

User rejects ExitPlanMode, explains the situation in the rejection message, and Claude has to fight plan mode restrictions to do any wrap-up work. It's confusing for both user and Claude.

Context

This is particularly important for users who use session rituals (updating context docs, extracting learnings, committing work) — these tasks need normal tool access, which plan mode blocks.