Plan files overwritten across concurrent sessions in same directory

Problem

When running multiple Claude Code sessions (terminals) in the same project directory, plan files in .claude/plans/ are shared and can be overwritten by other sessions.

Steps to reproduce:

1. Open two Claude Code terminals in the same project directory
2. Enter plan mode in Terminal A — creates a plan file (e.g., .claude/plans/snug-popping-wilkinson.md)
3. Enter plan mode in Terminal B — it sees Terminal A's plan and may modify/overwrite it due to the system instruction "If this plan is relevant to the current work and not already complete, continue working on it"
4. Terminal A's plan is now corrupted with Terminal B's context

Expected behavior: Each session's plans should be isolated. Session B should not see or modify Session A's active plan.

Actual behavior: All sessions in the same directory share .claude/plans/ with no session scoping. Plans from one session bleed into others.

Impact

This is a significant issue for power users running parallel sessions (e.g., one for backend work, one for frontend). The "continue working on this plan" instruction causes cross-contamination — Session B picks up Session A's plan and starts modifying it for a completely different task.

Suggested fix

Scope plan files by session ID, e.g., .claude/plans/{session-id}/plan-name.md, so concurrent sessions in the same directory don't interfere with each other.

Workaround

Run each session from a different subdirectory (e.g., cd backend vs cd frontend) so each gets its own .claude/plans/. This works but limits visibility of the full monorepo per session.