add interactive plan review to --plan mode

[umputun]

currently ralphex --plan presents three options after generating a plan draft: Accept, Revise, Reject. the "Revise" option asks for text feedback, which works but is limiting - you have to describe what to change instead of just editing the plan directly.

the idea: add a 4th option "Interactive review" that opens $EDITOR with the plan draft in a temp file. user edits directly, and on save+close ralphex computes a unified diff and feeds it back as revision feedback. closing without changes re-shows the menu. this gives a proper editing experience without requiring any specific terminal emulator.

why this matters:

• text-based revision feedback is imprecise ("add more details to task 3" vs just writing the details)
• direct editing is faster and more natural for plan refinement
• works everywhere $EDITOR works - no kitty/tmux/terminal-specific hacks needed since ralphex --plan owns its own terminal

approach:

• contained in pkg/input only - the runner does not care about feedback format, it just passes the string through
• editor lookup: $VISUAL then $EDITOR then vi (matches git convention)
• diff via go-difflib (already vendored through testify)
• editorFunc field on TerminalCollector for testability (same pattern as existing stdin/stdout fields)
• also fix bufio.Reader reuse - current AskDraftReview creates separate readers which will lose buffered data when wrapped in a loop

[umputun]

Implemented in PR #114 (merged).

• Added "Interactive review" as 4th option in the plan draft review menu
• Opens $EDITOR with plan content in a temp .md file
• On save, computes unified diff via go-difflib and wraps it with interpretation instructions
• Closing without changes re-shows the menu
• Editor lookup follows $VISUAL → $EDITOR → vi convention
• Fixed bufio.Reader threading to prevent data loss when looping
• editorFunc field on TerminalCollector for testability
• Updated make_plan.txt prompt to document the new option and diff-format feedback