…rl+C keeps draft

PR esengine#3051 split Shift+Tab (mode cycle) from Esc (cancel) by removing the
case m.bypass branch in the Esc handler, but missed the adjacent
case m.planMode branch sitting right next to it. Result: plan mode was
still escapable by hitting Esc, and the second unwanted behaviour fell
in the Ctrl+C branch: when both an active text selection and a
non-empty composer existed, Ctrl+C used to wipe the draft text while
also dropping the selection.

Two changes in internal/cli/chat_tui.go:

  1. Esc handler: drop the case m.planMode := false branch entirely.
     The mode-cycle switch (Shift+Tab) is now the single, documented
     way to toggle plan / YOLO. PR esengine#3051 already did the YOLO side;
     this commit finishes the plan side. The comment block above the
     switch is updated to match.

  2. Ctrl+C handler: hoist the selection-copy branch above the
     clear-input branch. Standard terminal convention: Ctrl+C copies
     the active selection. If we processed clear-input first we'd
     silently destroy the user's draft. After the copy a second
     Ctrl+C will still clear the composer; on an empty composer the
     double-press-quit gesture continues to work.

Tests (internal/cli/chat_tui_test.go):
  - TestEscInPlanModeDoesNotExitPlan: regression for the PR esengine#3051
    miss. The YOLO equivalent was already removed in esengine#3051 and is
    covered by control/yolo_test.go at the controller layer; not
    duplicated here.
  - TestCtrlCCopyBeatsClearInput: regression for the priority swap.
    Composer draft survives the copy; a follow-up Ctrl+C clears it.
  - Pre-existing TestCtrlCCopySelection and TestCtrlCClearsThen...
    continue to pass.