feat(webmcp): let agents drive the app instead of posting behind it - #12

Why

Every WebMCP tool posted straight to /api/*, so nothing on screen moved. An
agent approving an expense left the user looking at a row that still read
"Submitted". ToolRegistry.observe() refreshes only Session.pendingApprovals,
whose sole consumer is ToolSession gating a tool — no page reads it — and
every page's resource() params is signal-free, so no signal change could
re-trigger a load either.

A human cannot add an expense without going to /add, or change a budget
without going to /budgets. Now neither can an agent.

What

Writes go through the page. submit_expense, approve_expense and the new
set_budget live in tools/page-driven-tools.ts, which injects no
ApiClient. Each navigates to the owning page, waits for it to mount, and
hands over the arguments. The page performs the work through the same method its
own buttons call — so the optimistic row patching, the form messages, the
validation and the reloads all apply with no new plumbing.

set_budget is new. POST/PATCH /budgets and the create/edit form both
already existed; only the tool was missing, so an agent could read a budget
with get_budget_status and never change one.

navigate_to moves the browser between the seven authenticated pages. Its
enum descriptions double as the map of the app an agent reads straight off
getTools(), instead of reading the DOM and guessing where to click.

The Cambiaro converter is now a card on the dashboard, open on arrival. A
cross-origin tool lives only as long as the document that registered it, so a
collapsed frame on the landing screen meant the Copilot had no convertCurrency
until someone clicked. One mount only — two frames would publish the tool twice.

Details worth a reviewer's attention

• PageActions has no API fallback on timeout, deliberately. A fallback
  would restore exactly the invisible path this removes, and only when something
  went wrong — a slow chunk, a guard redirect. A timeout is an error the model
  reports.
• Unregister clears only the handler it installed. Angular builds the
  incoming component before destroying the outgoing one, so an unconditional
  delete lets a page being torn down wipe the incoming page's registration.
• The /add handler does not use form.requestSubmit(). onSubmit stamps
  the audit row actor: agentInvoked ? 'agent' : 'human', and a synthetic
  submit carries no agentInvoked — it would file the agent's work as a
  person's. That flag is the only thing in the app that can produce
  actor: 'human', and the audit viewer's contrast is built on it.
• The fill is paced (core/agent/fill-pacing.ts). Filling and submitting in
  one frame leaves no frame in which the filled form is on screen — identical to
  the invisible POST it replaced. Specs drive the stagger at 0.
• The Copilot collapses to the orb below sm, where the panel is
  fixed inset-0 and would hide the page it is driving. Collapses for the turn,
  not per call.
• submit_expense still means create and submit for approval; the
  declarative add_expense_form creates a draft only, so the handler performs
  the transition rather than quietly narrowing the contract.
• The declarative form's annotations are untouched.

Verification

1,051 tests pass — 12 shared, 149 backend, 890 frontend (34 new). Clean
production build and prerender. App hydrates with no console errors after
pnpm run dev:clean (required: SET_BUDGET is a new @actuo/shared export).

Not yet verified: the signed-in walkthrough — logging an expense, approving
it, and setting a budget through the Copilot, plus the mobile collapse at
< 640px.