feat(moments): the moment catalog rules will mount on, readable in-session (milestone 458 step 1, #4919)

Fourteen generic moments of work (session.start, work.start … reply.ask)
plus the skill.<name> family, each with what it means and the kinds of
action that reach it, written for any kind of work rather than software
alone. The catalog is code because every install needs the same mount
points; which actions reach a moment is per-install data (step 2).

require_moment refuses an unknown name with the catalog listed, so a
typo cannot become a mount that never fires. list_moments (read-only)
and GET /api/retrieval/moments hand out the same catalog.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/scribe/mcp/server.py

@@ -154,6 +154,11 @@ _READ_ONLY_TOOLS = frozenset({
     # most: it is the surface the "ask before acting" reflex calls, and a key
     # that could not reach it would be denied exactly the check it should run.
     "what_might_apply",
+    # The moment catalog (milestone 458). A pure read of a constant; the
+    # mount and mapping writes that use it are separate tools. Spelled out for
+    # retrieval_telemetry's reason, and needed by a read key so that a line
+    # naming a moment can be understood by whoever was shown it.
+    "list_moments",
 })
 
 # Every tool that WRITES, by name. Nothing reads this set at runtime — a tool