feat: /save exports the current session as json, md, or html on all platforms

Rework of salvaged PR #6372 (@ag9920) onto current main:

- /save promoted from CLI-only JSON snapshot to a cross-platform session
  export: `/save [json|md|html] [filename] [redact]` on CLI and every
  gateway platform (sent as a document via adapter.send_document).
- Rendering routes through the existing shared renderers
  (hermes_cli/session_export.py + session_export_html.py) instead of the
  PR's new hermes_state formatter — new helpers normalize_save_format /
  render_session_for_save / default_save_filename are shared by both
  surfaces.
- `redact` arg runs the export through the force-mode secret redaction
  pass (session_export_md.redact_session_data) before writing.
- Gateway handler awaits AsyncSessionDB correctly, sanitizes user-supplied
  filenames with basename, and lands in gateway/slash_commands.py (the
  handlers moved out of gateway/run.py since the PR was authored).
- /export stays profile export (name collision resolved: session export
  lives on /save).
- Slack 50-slash cap curation: /platform moves to the /hermes-only set to
  free a native slot for /save (parity test updated rationale comment).
- Folds in PR #62268 (@briandevans): None title/model coalescing in the
  single-session HTML export.

Closes #4249. Closes #51200.

hermes_state_portability.py

@@ -291,42 +291,6 @@ class SessionPortabilityMixin:
         base["messages"] = [msg for seg in segments for msg in (seg.get("messages") or [])]
         return base
 
-    @staticmethod
-    def format_session_as_markdown(export_data: Dict[str, Any]) -> str:
-        """Format exported session data as a readable Markdown document."""
-        lines = []
-        session_id = export_data.get("id", "Unknown")
-        lines.append(f"# Session Export: {session_id}\n")
-        
-        title = export_data.get("title")
-        if title:
-            lines.append(f"**Title**: {title}")
-
-        created_at = export_data.get("created_at")
-        if created_at:
-            lines.append(f"**Started**: {created_at}")
-
-        if title or created_at:
-            lines.append("\n---\n")
-
-        for msg in export_data.get("messages", []):
-            role = msg.get("role", "unknown").capitalize()
-            content = msg.get("content", "")
-
-            if role == "User":
-                lines.append(f"## {role}\n\n{content}\n")
-            elif role == "Assistant":
-                reasoning = msg.get("reasoning", "")
-                if reasoning:
-                    lines.append(f"## {role} (Reasoning)\n\n{reasoning}\n")
-                lines.append(f"## {role}\n\n{content}\n")
-            elif role == "Tool":
-                lines.append(f"### Tool Result\n\n```\n{content}\n```\n")
-            else:
-                lines.append(f"## {role}\n\n{content}\n")
-
-        return "\n".join(lines)
-
     def export_all(self, source: str = None) -> List[Dict[str, Any]]:
         """
         Export all sessions (with messages) as a list of dicts.