simplify(agent): drop the unused public Agent.abort(), keep whenIdle()

The public Agent handle exposed abort() (step-only) and cancel() (queue-aware).
No production caller used abort() — ACP maps session/cancel to cancel(), and
lifecycle owners tear down via AgentHandle.dispose(); the loop's own stop paths
abort their per-step AbortController directly. So abort() is latent generality
that keeps a private loop mechanic public.

RFC-premise correction: the public-agent-stop-surface RFC proposed removing
whenIdle() too. Implementation found whenIdle() load-bearing — a real
quiescence primitive with a deliberate loop contract (settle-without-transition,
the replacement-turn race) and ACP test consumers; its proposed replacement
("observe the running->idle transition") is exactly the async-state race
AGENTS.md warns against. So only abort() is removed; whenIdle() stays. The RFC
is amended on the way to implemented/ to record the narrowed scope, and the new
AGENTS.md "RFCs are proposals, not golden truth" principle (PR1) gets its
worked example.

- Remove Agent.abort() from the interface + the ReactLoopAgent impl; the no-arg
  'aborted' default goes with it (cancel() keeps its 'cancelled' default).
- Migrate tests: empty-queue abort() -> cancel(reason); the two review-fixes
  tests whose subject is the in-flight step's AbortController drive that
  controller directly via the private currentAbort field (cancel() would clear
  the inbox and destroy the queued steering one of them proves survives a step
  abort). The no-arg-default test is dropped (cancel()'s default is already
  covered in cancel.spec.ts).
- Resulting public stop surface: cancel() + whenIdle(). Update agent/agent-loop
  READMEs, architecture.md, core.md type-equiv, the extension cookbook, the
  lifecycle RFC (short note), and the proposed ACP RFC.

Implements docs/rfc/implemented/simplification/2026-06-20-public-agent-stop-surface.md

packages/core/agent-loop/tests/loop.spec.ts

@@ -318,7 +318,7 @@ describe('agent loop', () => {
     expect(adapter.requests[0]!.model).toBe('other-model')
   })
 
-  it('abort() mid-stream ends the turn with reason aborted', async () => {
+  it('cancel() mid-stream ends the turn with reason aborted', async () => {
     const adapter = new MockAdapter(['hang'])
     const ctx = await harness(adapter)
     const agent = ctx.agentLoop.create(AgentId('a1'), { model: 'mock' })
@@ -327,10 +327,10 @@ describe('agent loop', () => {
     ctx.on('agent/turn-end', (_agent, _turn, reason) => void reasons.push(reason))
 
     send(agent, 'go')
-    // wait until the stream is hanging, then abort
+    // wait until the stream is hanging, then cancel
     await new Promise(r => setTimeout(r, 30))
     expect(agent.status).toBe('running')
-    agent.abort('user interrupt')
+    agent.cancel('user interrupt')
     await waitForIdle(ctx, agent)
 
     expect(reasons).toEqual([{ kind: 'aborted', reason: 'user interrupt' }])