fix(skills): pin protects against deletion only, not edits (#20220)

Previously, pinning a skill blocked every skill_manage write action
(edit, patch, delete, write_file, remove_file). The 'hard fence'
design conflated two concerns:

  1. Pin as deletion protection — don't let the curator archive
     or the agent delete a stable skill.
  2. Pin as content freeze — don't let the agent rewrite it mid-conversation.

In practice (1) is what users pin for: they want a skill to survive
curator passes. (2) created friction — agents finding a new pitfall
in a pinned skill had to ask the user to unpin, then the agent
patches, then the user re-pins. The dance discouraged skill
maintenance and pinned skills went stale.

This narrows the _pinned_guard to skill_manage(action='delete') only.
Patches, edits, and supporting-file writes go through on pinned
skills so the agent can keep improving them. The curator's own
pinned-skip behavior (agent/curator.py:271 for auto-archive,
line 349 for the LLM review prompt) is unchanged — curator still
never touches pinned skills.

Changes:
- tools/skill_manager_tool.py: remove _pinned_guard calls from
  _edit_skill, _patch_skill, _write_file, _remove_file; keep on
  _delete_skill. Updated _pinned_guard docstring and error message.
- tools/skill_manager_tool.py: updated skill_manage model-facing tool
  description to reflect the new semantic.
- website/docs/user-guide/features/curator.md: updated pinning
  section.
- tests/tools/test_skill_manager_tool.py: flipped refuses-pinned
  tests for edit/patch/write_file/remove_file into allowed-when-pinned;
  kept test_delete_refuses_pinned (strengthened assertion to check the
  'cannot be deleted' wording).

Closes #18354

tools/skill_manager_tool.py

@@ -137,14 +137,12 @@ def _containing_skills_root(skill_path: Path) -> Path:
 def _pinned_guard(name: str) -> Optional[str]:
     """Return a refusal message if *name* is pinned, else None.
 
-    Pinned skills are off-limits to the agent's skill_manage tool.  The only
-    way to modify one is for the user to unpin it via
-    ``hermes curator unpin <name>`` (or edit it directly by hand).  This
-    mirrors the curator's own pinned-skip behavior but extends the guard
-    to tool-driven writes as well, giving users a hard fence against
-    accidental agent edits.
+    Pin protects a skill from **deletion** — both the curator's auto-archive
+    passes and the agent's ``skill_manage(action="delete")`` tool call. The
+    agent can still patch/edit pinned skills; pin only guards against
+    irrecoverable loss, not against content evolution.
 
-    Best-effort: if the sidecar is unreadable we let the write through
+    Best-effort: if the sidecar is unreadable we let the delete through
     rather than block on a broken telemetry file.
     """
     try:
@@ -152,9 +150,11 @@ def _pinned_guard(name: str) -> Optional[str]:
         rec = skill_usage.get_record(name)
         if rec.get("pinned"):
             return (
-                f"Skill '{name}' is pinned and cannot be modified by "
+                f"Skill '{name}' is pinned and cannot be deleted by "
                 f"skill_manage. Ask the user to run "
-                f"`hermes curator unpin {name}` if they want the change."
+                f"`hermes curator unpin {name}` if they want to delete it. "
+                f"Patches and edits are allowed on pinned skills; only "
+                f"deletion is blocked."
             )
     except Exception:
         logger.debug("pinned-guard lookup failed for %s", name, exc_info=True)
@@ -439,10 +439,6 @@ def _edit_skill(name: str, content: str) -> Dict[str, Any]:
     if not existing:
         return {"success": False, "error": f"Skill '{name}' not found. Use skills_list() to see available skills."}
 
-    pinned_err = _pinned_guard(name)
-    if pinned_err:
-        return {"success": False, "error": pinned_err}
-
     skill_md = existing["path"] / "SKILL.md"
     # Back up original content for rollback
     original_content = skill_md.read_text(encoding="utf-8") if skill_md.exists() else None
@@ -483,10 +479,6 @@ def _patch_skill(
     if not existing:
         return {"success": False, "error": f"Skill '{name}' not found."}
 
-    pinned_err = _pinned_guard(name)
-    if pinned_err:
-        return {"success": False, "error": pinned_err}
-
     skill_dir = existing["path"]
 
     if file_path:
@@ -645,10 +637,6 @@ def _write_file(name: str, file_path: str, file_content: str) -> Dict[str, Any]:
     if not existing:
         return {"success": False, "error": f"Skill '{name}' not found. Create it first with action='create'."}
 
-    pinned_err = _pinned_guard(name)
-    if pinned_err:
-        return {"success": False, "error": pinned_err}
-
     target, err = _resolve_skill_target(existing["path"], file_path)
     if err:
         return {"success": False, "error": err}
@@ -683,10 +671,6 @@ def _remove_file(name: str, file_path: str) -> Dict[str, Any]:
     if not existing:
         return {"success": False, "error": f"Skill '{name}' not found."}
 
-    pinned_err = _pinned_guard(name)
-    if pinned_err:
-        return {"success": False, "error": pinned_err}
-
     skill_dir = existing["path"]
 
     target, err = _resolve_skill_target(skill_dir, file_path)
@@ -835,9 +819,10 @@ SKILL_MANAGE_SCHEMA = {
         "Skip for simple one-offs. Confirm with user before creating/deleting.\n\n"
         "Good skills: trigger conditions, numbered steps with exact commands, "
         "pitfalls section, verification steps. Use skill_view() to see format examples.\n\n"
-        "Pinned skills are off-limits — all write actions refuse with a message "
-        "pointing the user to `hermes curator unpin <name>`. Don't try to route "
-        "around this by renaming or recreating."
+        "Pinned skills are protected from deletion only — skill_manage(action='delete') "
+        "will refuse with a message pointing the user to `hermes curator unpin <name>`. "
+        "Patches and edits go through on pinned skills so you can still improve them as "
+        "pitfalls come up; pin only guards against irrecoverable loss."
     ),
     "parameters": {
         "type": "object",