feat: /copy-plan command or shortcut to copy active plan to clipboard

Problem

In plan mode, the plan file lives at a generated path like ~/.claude/plans/vectorized-wiggling-pumpkin.md. To copy its content, users must either remember the path and run cat ... | pbcopy, or use the Read tool. There's no quick way to grab the plan content.

Requested

One of:

• A /copy-plan command that copies the active plan file to clipboard
• Make /copy context-aware — if in plan mode, copy the plan content instead of the last message
• A keybinding (e.g. Ctrl+Shift+C) that copies the active plan

Why it matters

Plan mode is iterative (audit → fix → resubmit). Users need to frequently share/review the plan outside Claude Code (paste into docs, send to colleagues, feed to another audit tool). The current workflow requires knowing the generated file path, which is friction-heavy when you're deep in an audit loop.

Current workaround

cat ~/.claude/plans/<generated-name>.md | pbcopy

This requires remembering or looking up the plan file path every time.