Plan mode context reset: unclear session handoff message

Summary

When using plan mode's "drop context and start fresh" feature, the new session's logs start with a misleading first message that doesn't indicate what happened. This makes it difficult to trace session lineage and understand why a session started the way it did.

Current behavior

After planning, Claude Code offers to drop context for a fresh implementation session. When accepted, the new session's transcript begins with:

1. [Request interrupted by user for tool use]
2. Implement the following plan: <plan>...

The first message is confusing — it reads like the user manually interrupted something, not like Claude Code's own plan-mode handoff feature was used.

Expected behavior

The first message in the new session should clearly indicate:

1. That this session was created via the plan mode "start fresh" feature
2. A reference to the previous session ID so the planning context can be traced back

Something like:

[New session created from plan mode handoff — previous session: abc123-def456]

Why this matters

• Session debugging: When reviewing logs or transcripts, there's no way to tell this session was spawned from a plan-mode reset vs. a manual interruption.
• Traceability: The planning session contains the rationale, exploration, and decisions that led to the plan. Without a link back, that context is disconnected.
• Hooks/automation: Anyone using SessionStart hooks or log analysis tooling can't distinguish plan handoffs from other session starts.