Rename session from /resume session list

[svkozak]

Summary

Recent update introduced /name command to add session names and this PR adds renaming support to the interactive /resume picker allowing to rename sessions in the list without opening them individually. Also modifies how session 'recency' is derived (if only metadata changed, session's 'modified' date remains the same).

Interactive /resume rename support

• Ctrl+N opens a full-page rename mode inside the session selector (no overlay).
• Rename mode shows “Rename Session”, an input, and save/cancel hints.
• Esc/Ctrl+C cancels rename; Enter saves.

Get session 'modified' time based on latest activity

• Session “modified” time is derived from last user/assistant message timestamp (with fallbacks), not file mtime, so renaming (or /name) doesn’t reorder the recent list. (mtime is kept as a fallback)

Refresh/reload behaviour

• After rename/delete, the session list refreshes so updated names appear immediately.
• Small refactor of refresh logic into a shared loadScope(...) helper to reduce duplication.

Tests

• Added/updated tests to cover:
  • rename hint behavior (/resume vs --resume)
  • rename flow (Ctrl+N → rename UI → typing → Enter)
  • “modified timestamp uses last message timestamp” behavior
  • existing delete/path/scope behavior remains covered