feat(gateway,cli): confirm /reload-mcp to warn about prompt cache invalidation

Reloading MCP servers rebuilds the tool set for the active session, which
invalidates the provider prompt cache (tool schemas are baked into the
system prompt). The next message re-sends full input tokens — can be
expensive on long-context or high-reasoning models.

To surface that cost, /reload-mcp now routes through a new slash-confirm
primitive with three options: Approve Once / Always Approve / Cancel.
'Always Approve' persists approvals.mcp_reload_confirm: false so future
reloads run silently.

Coverage:

* Classic CLI (cli.py) — interactive numbered prompt.
* TUI (tui_gateway + Ink ops.ts) — text warning on first call; `now` /
  `always` args skip the gate; `always` also persists the opt-out.
* Messenger gateway — button UI on Telegram (inline keyboard), Discord
  (discord.ui.View), Slack (Block Kit actions); text fallback on every
  other platform via /approve /always /cancel replies intercepted in
  gateway/run.py _handle_message.
* Config key: approvals.mcp_reload_confirm (default true).
* Auto-reload paths (CLI file watcher, TUI config-sync mtime poll) pass
  confirm=true so they do NOT prompt.

Implementation:

* tools/slash_confirm.py — module-level pending-state store used by all
  adapters and by the CLI prompt. Thread-safe register/resolve/clear.
* gateway/platforms/base.py — send_slash_confirm hook (default 'Not
  supported' → text fallback).
* gateway/run.py — _request_slash_confirm helper + text intercept in
  _handle_message (yields to in-progress tool-exec approvals so
  dangerous-command /approve still unblocks the tool thread first).

Tests:

* tests/tools/test_slash_confirm.py — primitive lifecycle + async
  resolution + double-click atomicity (16 tests).
* tests/hermes_cli/test_mcp_reload_confirm_gate.py — default-config
  shape + deep-merge preserves user opt-out (5 tests).

Targeted runs (hermetic): 89 passed (slash-confirm, config gate,
existing agent cache, existing telegram approval buttons).

gateway/platforms/base.py

@@ -1415,6 +1415,41 @@ class BasePlatformAdapter(ABC):
         """
         return False
 
+    async def send_slash_confirm(
+        self,
+        chat_id: str,
+        title: str,
+        message: str,
+        session_key: str,
+        confirm_id: str,
+        metadata: Optional[Dict[str, Any]] = None,
+    ) -> SendResult:
+        """Send a three-option slash-command confirmation prompt.
+
+        Used by the gateway's generic slash-confirm primitive (see
+        ``GatewayRunner._request_slash_confirm``) for commands that have a
+        non-destructive but expensive side effect the user should explicitly
+        acknowledge — the current caller is ``/reload-mcp``, which
+        invalidates the provider prompt cache.
+
+        Platforms with inline-button support (Telegram, Discord, Slack,
+        Matrix, Feishu) should override this to render three buttons:
+        Approve Once / Always Approve / Cancel.  Button callbacks MUST be
+        routed back through the gateway by calling
+        ``GatewayRunner._resolve_slash_confirm(confirm_id, choice)`` where
+        ``choice`` is ``"once"`` / ``"always"`` / ``"cancel"``.
+
+        Platforms without button UIs leave this as the default and fall
+        through to the gateway's text fallback (which sends ``message`` as
+        plain text and intercepts the next ``/approve`` / ``/always`` /
+        ``/cancel`` reply).
+
+        ``confirm_id`` is a short string generated by the gateway; the
+        adapter stores it alongside any platform-specific state needed to
+        route the callback (e.g. Telegram's ``_approval_state`` dict).
+        """
+        return SendResult(success=False, error="Not supported")
+
     async def send_typing(self, chat_id: str, metadata=None) -> None:
         """
         Send a typing indicator.