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).

hermes_cli/config.py

@@ -1018,6 +1018,14 @@ DEFAULT_CONFIG = {
         "mode": "manual",
         "timeout": 60,
         "cron_mode": "deny",
+        # When true, /reload-mcp asks the user to confirm before rebuilding
+        # the MCP tool set for the active session.  Reloading invalidates
+        # the provider prompt cache (tool schemas are baked into the system
+        # prompt), so the next message re-sends full input tokens — this can
+        # be expensive on long-context or high-reasoning models.  Users click
+        # "Always Approve" to silence the prompt permanently; that flips
+        # this key to false.
+        "mcp_reload_confirm": True,
     },
 
     # Permanently allowed dangerous command patterns (added via "always" approval)