fix(tui): clipboard copy on linux/wayland

`probeLinuxCopy` and `copyNative` in `osc.ts` await `execFileNoThrow`
for wl-copy / xclip / xsel. Those tools double-fork a daemon that
holds the system selection live, and the daemon inherits stdio pipes
from `spawn(stdio: 'pipe')`. Node's 'close' event only fires when
stdio is fully closed → the daemon keeps the pipes open → 'close'
never fires → the await leaks past the timeout (kill(SIGTERM) on an
already-exited child is a no-op, daemon survives).

Result: `linuxCopy` cache stays `undefined` permanently, the actual
copy never runs, ctrl-c silently does nothing on wayland/x11.
Reproduced in isolation, confirmed across wl-copy and a
daemonization-shaped fixture.

Fix: add `resolveOnExit` option to `execFileNoThrow`. When set, the
promise settles on the immediate child's 'exit' event instead of
waiting for stdio drainage. Wired into both the probe and the actual
copy spawns for every clipboard tool (pbcopy, wl-copy, xclip, xsel,
clip).

Tests: 5 new vitest cases covering daemon-style child handling,
non-zero exit propagation, timeout behavior, and double-resolve
guard. The forever-hang case is committed as `it.skip` with
documentation so a reviewer can verify the bug by hand.

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

@@ -345,7 +345,7 @@ export const coreCommands: SlashCommand[] = [
           return sys(`copied ${text.length} characters`)
         } else {
           return sys(
-            'clipboard copy failed — try HERMES_TUI_FORCE_OSC52=1 to force the escape sequence; HERMES_TUI_DEBUG_CLIPBOARD=1 for details'
+            'clipboard copy failed — try HERMES_TUI_FORCE_OSC52=1 to force the escape sequence'
           )
         }
       }