[BUG/FEATURE] Plan files grow indefinitely, burying current work under historical context

Problem

When working on a long-running project, plan files accumulate all historical context indefinitely. After a few weeks of active development, my plan file is now 2400+ lines, containing:

• Completed phases from weeks ago
• Old design decisions that are no longer relevant
• Investigation notes from resolved issues
• Current work (buried somewhere in the middle)

This makes the plan file essentially unusable for its intended purpose - understanding "what am I currently trying to do?"

Steps to Reproduce

1. Start a new project, enter plan mode
2. Work on Feature A, document the plan
3. Complete Feature A
4. Enter plan mode for Feature B - same file, content appended
5. Repeat for Features C, D, E...
6. After ~10 planning sessions, the file becomes unwieldy

Current Behavior

• All planning sessions append to the same file per project
• Completed work remains in the file forever
• Claude adds new sections but never archives/removes old ones
• When re-entering plan mode, Claude reads the entire bloated file

Expected Behavior

One of:

1. Session-based plan files - New plan file for each planning session, with option to reference previous
2. Automatic archival - Move completed sections to an archive file
3. Plan file rotation - When file exceeds N lines, archive and start fresh
4. Explicit lifecycle - Let users mark sections as "done" to be archived

Environment

• Claude Code version: 2.1.20
• OS: macOS

Additional Context

Related issues that touch on plan file UX but don't address this specific problem:

• #21342 (file naming)
• #20899 (response buried)
• #18434 (lifecycle documentation)