fix(curator): protect load-bearing built-in skills from archival/consolidation (#41817)

The curator's idle-archival path (apply_automatic_transitions under
prune_builtins) could archive the bundled `plan` skill, killing the
/plan slash command silently — typing /plan then returned 'Unknown
command' with no signal that a skill had vanished. The archived skill's
hash stays in .bundled_manifest, so 'hermes update' wouldn't re-seed it.

Add PROTECTED_BUILTIN_SKILLS ({plan}) enforced at the master gate
is_curation_eligible() (covers archive_skill + the transition walk) and
in the candidate enumerator (so the LLM consolidation pass never sees
them). Immune to prune_builtins, pin state, and LLM judgment.

agent/curator.py

@@ -375,6 +375,11 @@ CURATOR_REVIEW_PROMPT = (
     "into ~/.hermes/skills/.archive/) is the maximum destructive action. "
     "Archives are recoverable; deletion is not.\n"
     "3. DO NOT touch skills shown as pinned=yes. Skip them entirely.\n"
+    "3b. DO NOT archive, delete, consolidate, move, or otherwise modify any "
+    "skill named in the protected built-ins list (currently: plan). These "
+    "back load-bearing UX (slash-command entry points referenced in docs and "
+    "tips) and are filtered out of the candidate list below — never resurrect "
+    "one as an archive or absorb target.\n"
     "4. DO NOT use usage counters as a reason to skip consolidation. The "
     "counters are new and often mostly zero. Judge overlap on CONTENT, "
     "not on use_count. 'use=0' is not evidence a skill is valuable; it's "