refactor(brief): move the action lifecycle into the machine (RD-12)

The action lifecycle (Idle | Busy | Failed) lived in an imperative
store-level signal, set from ten call sites outside the reducer. The
reducer could not enforce which action transitions are legal.

Add `action` to `BriefState.Loaded`, driven by three new messages
(ActionStarted, ActionFinished, ActionFailed) and handled in `reduce`.
Replace every `actionState.set(...)` call in `brief.store.ts` with the
matching `dispatch`. `BriefLoaded` resets `action` to Idle, so a fresh
load clears a stale action error instead of letting it outlive the
reload.

`busy` and `lastError` stay as `computed`s on the store with a
byte-identical public signature — they are the render seam for four
components and two page templates, and the union belongs in the
machine, not the components. `revealBigNummer` still sets only
`Failed`, never `Busy` — an existing asymmetry, not changed here.
`SaveState`, `org-template.store.ts`, and `pendingPublish` are out of
scope (RD-13, RD-14).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

libs/shared/docs/behaviour-spec.mdx

@@ -20,7 +20,7 @@ tested where._
 
 Every bullet below is a real test name from the suite — an `it()` title (frontend) or a test
 method name (backend), read as a sentence. Nothing here is hand-written prose: this page
-**is** the suite, reshaped for a business reader. 519 frontend behaviours across
+**is** the suite, reshaped for a business reader. 524 frontend behaviours across
 9 contexts; 261 backend behaviours across 42 test
 classes.
 
@@ -260,6 +260,11 @@ classes.
 - approve fires only from submitted
 - reject fires from submitted, carrying comments
 - send fires only from approved
+- ActionStarted moves a loaded brief to Busy
+- ActionFailed carries the error
+- ActionFinished returns to Idle
+- BriefLoaded resets a stale action error to Idle
+- an action message is a no-op when the brief is not loaded
 - a status transition replaces decisions with the fresh server value
 
 #### diffBlocks