feat(moments): the human door onto mounts, mappings and per-moment telemetry (milestone 458 step 6, #4924)

Everything an agent can do with moments, a person can now see and change
in the app.

- Rule editor: a moment picker beside the trigger. Catalog moments are
  ticked; a named procedure's `skill.<name>` is typed and checked as the
  server checks it. `moments` is always sent, so unticking the last moment
  unmounts the rule.
- Settings, Moments section (General tab): for each moment, what it
  means, the actions that reach it on this install (shipped ones can be
  switched off, the install's own removed), how many rules are mounted on
  it, and deliveries and agent opens over the window. Below that: named
  procedures with mounts, switched-off defaults with Restore, and a form to
  add an action.
- retrieval_telemetry.moment_usage: per moment, `delivered`, `rules`,
  `opened` (agent pulls after the first delivery there; an upper bound, as
  by_source is) and `last_delivered_at`. No ratio, because a mount is a
  person's statement, not a ranker's guess. Guarded on its own, and also
  reported in retrieval_summary as `moment_usage`.
- rulebooks.mount_counts; mounted_moments now derives from it.
- GET /api/retrieval/moments carries `mounted` and `usage` (?days=).
  DELETE /moments/mappings also reads the mapping from query parameters,
  since the browser's DELETE sends no body.

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

frontend/src/api/rulebooks.ts

@@ -82,6 +82,9 @@ export interface Rule {
   updated_at: string | null;
   /** Present only when the rule has them (the server omits empty keys). */
   systems?: { id: number; name: string }[];
+  /** The moments this rule arrives at whenever they happen (milestone 458),
+   *  in catalog order. Present only when the rule is mounted. */
+  moments?: string[];
   relations?: RuleRelation[];
   /** The lessons that point at this rule (milestone 440) — the concrete
    *  situations judged instances of it, plus any suggested and awaiting a
@@ -210,6 +213,8 @@ export interface RuleWrite {
   how_to_apply: string;
   order_index: number;
   system_ids: number[];
+  /** Replaces the set; [] unmounts the rule from every moment. */
+  moments: string[];
   arose_from_id: number | null;
   verify_with: string;
   expires_when: string;