fix(skills): exclude .archive from skill index walk

Archived skills (moved to ~/.hermes/skills/.archive/ by the curator)
were still surfaced in the <available_skills> system prompt under a
fake '.archive' category, causing the agent to load and try to use
deprecated skills. The os.walk in iter_skill_index_files() only
excluded .git/.github/.hub.

Add '.archive' to EXCLUDED_SKILL_DIRS, and to the two other places
that hardcode the same exclusion tuple (gateway/run.py and
agent/skill_commands.py).

gateway/run.py

@@ -638,7 +638,7 @@ def _check_unavailable_skill(command_name: str) -> str | None:
             if not skills_dir.exists():
                 continue
             for skill_md in skills_dir.rglob("SKILL.md"):
-                if any(part in ('.git', '.github', '.hub') for part in skill_md.parts):
+                if any(part in ('.git', '.github', '.hub', '.archive') for part in skill_md.parts):
                     continue
                 name = skill_md.parent.name.lower().replace("_", "-")
                 if name == normalized and name in disabled: