fix(curator): only mark agent-created for background-review sediment (#19621)

Tighten the provenance semantics added in #19618: skills a user asks a
foreground agent to write via skill_manage(create) now stay invisible to
the curator. Only skills the background self-improvement review fork
sediments through skill_manage get the created_by=agent marker.

- tools/skill_provenance.py — new ContextVar module mirroring the
  _approval_session_key pattern: set_current_write_origin / reset /
  get / is_background_review. Default origin is 'foreground'; the
  review fork sets 'background_review'.
- run_agent.py — run_conversation() binds the ContextVar from
  self._memory_write_origin at the top of each call. The review fork
  runs on its own thread (fresh context), so foreground and review
  contexts never cross-contaminate.
- tools/skill_manager_tool.py — skill_manage(action='create') now
  only calls mark_agent_created() when is_background_review(). All
  other cases (foreground create, patch, edit, write_file, delete)
  continue as before.
- tests: test_skill_provenance.py (6 tests covering the ContextVar
  surface), split test_full_create_via_dispatcher into foreground
  vs. review-fork variants, curator status tests now mark-first.

Why: the agent routinely edits existing user skills on the user's
behalf; those writes must never flip provenance. And when a user
explicitly asks the foreground agent to create a skill, that skill
belongs to the user. The curator should only be cleaning up after
its own autonomous sediment from the review nudge loop.

tools/skill_provenance.py

@@ -0,0 +1,78 @@
+"""Skill write-origin provenance — ContextVar for distinguishing agent-sediment skill writes from foreground user-directed writes.
+
+The curator only consolidates/prunes skills it autonomously created via the
+background self-improvement review fork. Skills a user asks a foreground
+agent to write belong to the user and must never be auto-curated.
+
+This module exposes a ContextVar that run_agent.py sets before each tool
+loop so tool handlers (e.g. skill_manage create) can check whether they
+are executing inside the background-review fork.
+
+The signal piggybacks on AIAgent._memory_write_origin, which is already
+set to "background_review" for review-fork instances (see
+_spawn_background_review in run_agent.py) and defaults to "assistant_tool"
+for normal (foreground) agents.
+
+Usage:
+    from tools.skill_provenance import (
+        set_current_write_origin,
+        reset_current_write_origin,
+        get_current_write_origin,
+    )
+
+    token = set_current_write_origin("background_review")
+    try:
+        ...  # tool runs here
+    finally:
+        reset_current_write_origin(token)
+
+    # inside a tool:
+    if get_current_write_origin() == "background_review":
+        mark_agent_created(skill_name)
+"""
+
+import contextvars
+
+
+_write_origin: contextvars.ContextVar[str] = contextvars.ContextVar(
+    "skill_write_origin",
+    default="foreground",
+)
+
+# The sentinel value the background review fork uses; mirrors
+# run_agent.py's AIAgent._memory_write_origin override in
+# _spawn_background_review().
+BACKGROUND_REVIEW = "background_review"
+
+
+def set_current_write_origin(origin: str) -> contextvars.Token[str]:
+    """Bind the active write origin to the current context.
+
+    Returns a Token the caller must pass to reset_current_write_origin
+    in a finally block.
+    """
+    return _write_origin.set(origin or "foreground")
+
+
+def reset_current_write_origin(token: contextvars.Token[str]) -> None:
+    """Restore the prior write origin context."""
+    _write_origin.reset(token)
+
+
+def get_current_write_origin() -> str:
+    """Return the active write origin.
+
+    Default: "foreground" — any tool call made by a regular (non-review)
+    agent, from the CLI, the gateway, cron, or a subagent.
+
+    "background_review" — the self-improvement review fork; only skills
+    created under this origin should be marked agent-created for curator
+    management.
+    """
+    return _write_origin.get()
+
+
+def is_background_review() -> bool:
+    """Convenience: True iff the current write origin is the background
+    review fork."""
+    return get_current_write_origin() == BACKGROUND_REVIEW