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/tools/moments.py

@@ -0,0 +1,36 @@
+"""The moment catalog as an MCP tool (milestone 458 step 1).
+
+A session needs the vocabulary in hand to mount a rule, to correct which
+action reaches which moment, and to read a moment line it was shown — and it
+needs it without leaving the session for a settings page. So the catalog is a
+tool from the first step, ahead of the writes that will use it.
+"""
+from __future__ import annotations
+
+from scribe.services import moments as moments_svc
+
+
+async def list_moments() -> dict:
+    """The moments of work that rules mount on — their names and what each means.
+
+    A rule mounted on a moment arrives whenever that moment happens, whatever
+    the words of the work look like. That is how a rule reaches you when it is
+    about WHEN something is done rather than WHAT it is about: a rule on
+    finishing work belongs at `work.finish` and `reply.report`, where nothing
+    said needs to resemble it.
+
+    Read this when you are about to mount a rule, when a line you were shown
+    names a moment and you want its meaning, or when you are correcting which
+    of your actions reaches which moment. Names are `<area>.<verb>`; a named
+    procedure (a skill or a stored process) is its own moment,
+    `skill.<name>`, listed under `families`.
+
+    Each moment says what is happening at it (`means`) and the kinds of action
+    that typically reach it (`reached_by`). The actions are examples: which of
+    YOUR actions reach a moment varies by install and is corrected in-session.
+    """
+    return moments_svc.catalog()
+
+
+def register(mcp) -> None:
+    mcp.tool(name="list_moments")(list_moments)