Bring back 'clear context and implement' option after plan mode

Feature Request

After exiting plan mode (via ExitPlanMode), there used to be an option to clear the conversation context and proceed with implementation using the plan file as the sole context. This was extremely valuable for large implementation sessions because:

1. Plan mode exploration consumes significant context (reading many files, launching explore agents)
2. By the time implementation begins, the context window is often 50-90% consumed
3. The plan file captures everything needed for implementation — the exploration context is no longer needed
4. Starting fresh with just the plan file gives maximum context for the actual coding work

This option disappeared around the time of the Opus 4.6 (1m) release. The current behavior exits plan mode and continues in the same context, which means large plans often hit auto-compact during implementation.

Expected Behavior

After ExitPlanMode and user approval, offer an option like:

• "Clear context and implement from plan" — resets conversation, loads plan file, begins implementation
• "Continue in current context" — current behavior (keep all exploration context)

Environment

• Claude Code CLI (latest)
• Model: claude-opus-4-6
• The plan file is already written to disk (~/.claude/plans/...), so it survives context clearing

This was a genuinely useful workflow optimization — please bring it back.