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/agent.spec.ts

@@ -288,7 +288,7 @@ describe('ReactLoopAgent', () => {
     expect(settled).toBe(false)
 
     await waitForStatus(ctx, agent, 'running')
-    agent.abort('done')
+    agent.cancel('done')
     await idle
     expect(settled).toBe(true)
     expect(agent.status).toBe('idle')
@@ -430,20 +430,4 @@ describe('ReactLoopAgent', () => {
     expect(warn).toHaveBeenCalledWith(expect.stringContaining('agent/status listener threw on idle'))
     warn.mockRestore()
   })
-
-  it('abort() resolves reason to "aborted" when no reason provided', async () => {
-    const adapter = new MockAdapter(['hang'])
-    const ctx = await harness(adapter)
-    const agent = ctx.agentLoop.create(AgentId('a1'), { model: 'mock' })
-
-    const reasons: { kind: string; reason?: string }[] = []
-    ctx.on('agent/turn-end', (_agent, _turn, reason) => void reasons.push(reason))
-
-    send(agent, 'go')
-    await new Promise(r => setTimeout(r, 30))
-    agent.abort() // no reason string
-    await waitForIdle(ctx, agent)
-
-    expect(reasons[0]).toMatchObject({ kind: 'aborted', reason: 'aborted' })
-  })
 })

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' }])

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

@@ -92,7 +92,7 @@ describe('HIGH: session log records what agent/step-result actually produced', (
 })
 
 describe('HIGH: abort during tool execution ends the turn', () => {
-  it('abort() inside a tool prevents both remaining tools and the next model step', async () => {
+  it('aborting the in-flight step inside a tool prevents both remaining tools and the next model step', async () => {
     const adapter = new MockAdapter([
       // model asks for two tool calls in one step
       [
@@ -113,7 +113,11 @@ describe('HIGH: abort during tool execution ends the turn', () => {
       parameters: {},
       async execute() {
         executed.push('aborter')
-        agent.abort('user interrupt')
+        // Fire the in-flight step's AbortController directly (the loop registers
+        // it on the agent). This is the bare step-abort path — distinct from
+        // cancel(), which would also clear the inbox; here the subject is the
+        // loop's response to its running step being aborted mid-tool.
+        ;(agent as unknown as { currentAbort?: AbortController }).currentAbort?.abort('user interrupt')
         return [{ type: 'text', text: 'done' }]
       },
     }))
@@ -228,7 +232,12 @@ describe('HIGH: steering from late extension points is never stranded', () => {
     send(agent, 'go')
     await new Promise(r => setTimeout(r, 30))
     agent.steer([{ type: 'text', text: 'redirect' }])
-    agent.abort('user interrupt')
+    // Abort ONLY the in-flight step, via its AbortController directly — NOT
+    // cancel(), which clears the inbox and would drop the queued steering this
+    // test proves survives a step abort. There is no public step-only abort
+    // verb (cancel() is the only public stop primitive), so reach the private
+    // controller the loop registered.
+    ;(agent as unknown as { currentAbort?: AbortController }).currentAbort?.abort('user interrupt')
     await waitForIdle(ctx, agent)
 
     // a new turn ran with the steering content delivered as a message