feat(skills): stacked slash-skill invocations — /skill-a /skill-b do XYZ (#57987)

Inspired by Claude Code v2.1.199 (July 2, 2026): stacked slash-skill
invocations load all leading skills (up to 5), not just the first.

- agent/skill_commands.py: split_stacked_skill_commands() consumes leading
  /skill tokens (stops at the first non-skill token so slash-path arguments
  are never swallowed); build_stacked_skill_invocation_message() composes
  the multi-skill turn reusing the existing bundle scaffolding markers so
  extract_user_instruction_from_skill_message() keeps memory providers
  storing the user's instruction, not N skill bodies.
- cli.py + gateway/run.py: dispatch the stacked path on both surfaces.
- 11 new tests + docs section in skills.md.

cli.py

@@ -8890,7 +8890,39 @@ class HermesCLI(CLIAgentSetupMixin, CLICommandsMixin):
                     )
             # Check for skill slash commands (/gif-search, /axolotl, etc.)
             elif base_cmd in skill_commands:
-                user_instruction = cmd_original[len(base_cmd):].strip()
+                rest = cmd_original[len(base_cmd):].strip()
+                # Stacked slash-skill invocations: `/skill-a /skill-b do XYZ`
+                # loads every leading skill (up to 5), not just the first.
+                # Inspired by Claude Code v2.1.199.
+                from agent.skill_commands import (
+                    build_stacked_skill_invocation_message,
+                    split_stacked_skill_commands,
+                )
+                extra_keys, user_instruction = split_stacked_skill_commands(rest)
+                if extra_keys:
+                    stacked_result = build_stacked_skill_invocation_message(
+                        [base_cmd, *extra_keys],
+                        user_instruction,
+                        task_id=self.session_id,
+                    )
+                    if stacked_result:
+                        msg, loaded_names, missing = stacked_result
+                        print(
+                            f"\n⚡ Loading {len(loaded_names)} stacked skills: "
+                            f"{', '.join(loaded_names)}"
+                        )
+                        if missing:
+                            ChatConsole().print(
+                                f"[yellow]Skipped missing skills: {', '.join(missing)}[/]"
+                            )
+                        if hasattr(self, '_pending_input'):
+                            self._pending_input.put(msg)
+                    else:
+                        ChatConsole().print(
+                            f"[bold red]Failed to load stacked skills for {base_cmd}[/]"
+                        )
+                    return True
+                user_instruction = rest
                 msg = build_skill_invocation_message(
                     base_cmd, user_instruction, task_id=self.session_id
                 )