feat: confirm prompt for destructive slash commands (#4069) (#22687)

/clear, /new, /reset, and /undo now ask the user to confirm before
discarding conversation state — three-option prompt routed through the
existing tools.slash_confirm primitive.

Native yes/no buttons render on Telegram, Discord, and Slack (their
adapters already implement send_slash_confirm); other platforms get a
text-fallback prompt and reply with /approve, /always, or /cancel.

The classic prompt_toolkit CLI uses the same three-option flow via the
established _prompt_text_input pattern (see _confirm_and_reload_mcp).
TUI keeps its existing modal overlay (#12312).

Gated by new config key approvals.destructive_slash_confirm (default
true). Picking 'Always Approve' flips the gate to false so subsequent
destructive commands run silently — matches the established
mcp_reload_confirm UX.

Out of scope: /cron remove (separate domain — scheduled jobs, not
session history). Existing TUI overlay env-var (HERMES_TUI_NO_CONFIRM)
left unchanged; cosmetic unification can come later.

Closes #4069.

tests/cli/test_cli_new_session.py

@@ -130,6 +130,11 @@ def _prepare_cli_with_active_session(tmp_path):
     old_session_start = cli.session_start - timedelta(seconds=1)
     cli.session_start = old_session_start
     cli.agent.session_start = old_session_start
+
+    # Bypass the destructive-slash confirmation gate — these tests focus on
+    # the new-session mechanics, not the confirm prompt itself (covered in
+    # tests/cli/test_destructive_slash_confirm.py).
+    cli._confirm_destructive_slash = lambda *_a, **_kw: "once"
     return cli