fix(dispatcher): send a lone Enter after the task paste to submit it

Multi-line task bodies arrived in Claude Code as "[Pasted text #N +M lines]"
and sat in the input buffer forever — the trailing Enter that SendKeys
appends to the paste is consumed as a newline inside the paste, not as a
submit. Observed live on ccl-auto-11 (secumon) and ccl-auto-12 (secuops):
prompt visible, agent idle.

- tmux.Client grows a SendEnter(session) method. ExecClient runs
  `tmux send-keys -t <sess> Enter` (no preceding text), which Claude's
  TUI accepts as the explicit submit action after a paste.
- Dispatcher: after SendKeys(msg), sleep 500ms for the paste to register,
  then SendEnter. Same sequence a human would perform.
- Five mockTmux implementations updated (quota, dispatcher, switcher,
  lifecycle, watcher tests).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/quota/monitor_test.go

@@ -24,6 +24,7 @@ func (m *mockTmux) HasSession(name string) bool        { return m.sessions[name]
 func (m *mockTmux) CreateSession(name, _ string) error { m.sessions[name] = true; return nil }
 func (m *mockTmux) KillSession(_ string) error         { return nil }
 func (m *mockTmux) SendKeys(_, _ string) error         { return nil }
+func (m *mockTmux) SendEnter(_ string) error           { return nil }
 func (m *mockTmux) CapturePaneTail(session string, _ int) (string, error) {
 	return m.paneOutput[session], nil
 }