[Bug] ExitPlanMode opens blank approval dialog

Bug Description
Plan mode: ExitPlanMode opens a blank approval dialog when the session's plan file is missing. Confirmed twice in one session. Original incident: in plan mode the model wrote its plan to ~/.claude/plans/here-is-another-extra-nifty-treehouse.md. At 2026-09-17T18:38:33Z the harness wrote a "continued-in" record as the last line of transcript 4dbd3c73-6b60-4009-b2f9-b99ae7f74936.jsonl and continued the conversation in a new session beef1468-3abe-477b-be56-fa6ed367aa46. No user command, no agent event caused it. The plan path follows the session ID, so it silently became ~/.claude/plans/curious-noodling-planet.md, which did not exist. ExitPlanMode opened the approval dialog with no plan in it. The system message naming the new path only reached the model several turns later. Controlled repro: delete the current session's plan file, have the model call ExitPlanMode. Dialog opens in the same blank state. Expected: plan path stays fixed for a plan-mode session, or the model is told immediately when it changes and the existing plan is migrated. ExitPlanMode should refuse or warn on a missing plan file instead of opening a blank dialog. Environment: Claude Code 2.1.274 (native build), Linux 6.8.0-138-generic, zsh, model claude-fable-5-1. Three empty scratchpad dirs were created under /tmp/claude-1000/-home-bc-cng/ at the same second as the re-key. Earlier report receipt: 4d4fe2e0-69a0-4fff-b67f-97ff0c862f85.

Environment Info

• Platform: linux
• Terminal: xterm-256color
• Version: 2.1.274
• Feedback ID: 2fc069ac-6d43-4c00-9d26-4ff0e8e9cb63

Errors

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