fix(clipboard): report native/tmux success, keep Ctrl+Shift+C on dashboard

Follow-up on #16020 salvage. Three corrections:

1. Truth signal for /copy
   Before: success was 'OSC 52 sequence was emitted to stdout'. That's
   false on local Linux inside tmux (emitSequence=false), so /copy kept
   printing 'clipboard copy failed' to users whose xclip/wl-copy had
   already succeeded fire-and-forget.
   Fix: setClipboard() now returns { sequence, success } where success =
   native-fired OR tmux-buffer-loaded OR osc52-emitted. copyNative()
   returns a boolean telling setClipboard whether a native attempt was
   made. /copy only shows 'failed' when literally no path was taken.

2. Dashboard keybinding
   Before: Ctrl+C for copy on non-Mac (Ctrl+Shift+C for paste).
   That swallows SIGINT when a stale selection is present and breaks
   the xterm/gnome-terminal/konsole/Windows-Terminal convention where
   Ctrl+C in a terminal emulator is always SIGINT. The real bug was
   that clipboard writes lost user-gesture through OSC-52 round-trips,
   which the direct writeText already fixes.
   Fix: revert copyModifier to Ctrl+Shift+C on non-Mac. Direct
   writeText in the keydown handler preserves user gesture. term.write
   Escape replaced with term.clearSelection() (works without relying
   on TUI input mode).

3. Error toast text
   Before: 'see HERMES_TUI_DEBUG_CLIPBOARD' — tells users how to
   debug but not how to fix.
   Fix: point users at HERMES_TUI_FORCE_OSC52=1 first (the actual
   escape hatch), mention the debug var second.

ui-tui/src/app/slash/commands/core.ts

@@ -256,11 +256,11 @@ export const coreCommands: SlashCommand[] = [
 
       if (!arg && ctx.composer.hasSelection) {
         const text = await ctx.composer.selection.copySelection()
+
         if (text) {
-          // Include character count to match user's reported message format
           return sys(`copied ${text.length} characters`)
         } else {
-          return sys('clipboard copy failed — no OSC 52 emitted; see HERMES_TUI_DEBUG_CLIPBOARD')
+          return sys('clipboard copy failed — try HERMES_TUI_FORCE_OSC52=1 to force the escape sequence; HERMES_TUI_DEBUG_CLIPBOARD=1 for details')
         }
       }