✨ feat(acp): ACPX provider package (acp-stack 07)

[taras]

Why

The <TestAgent> phase (turning the manual ACPX walkthrough into an xmd test
fixture) sits on top of an ACP-client provider. #135 (merged) added the
provider-neutral core agent vocabulary and the rootProvider factory seam.
This PR adds packages/acp — the ACPX implementation of that seam,
connecting the core Agent Api to acpx@0.12.0. Library-only: no
<TestAgent> vocabulary, no CLI provider selection, no real-subprocess smoke —
those follow in the <TestAgent> phase.

What changes

• packages/acp (@executablemd/acp v0.4.2):
  • createAcpxProvider() — the AgentProviderFactory for
    installAgentVocabulary({ rootProvider }); useAcpxProviderState for the
    same operations without the Agent install (sibling states).
  • Agent availability via a probe runtime's doctor() (cached); session
    creation + routing by (agent, session key, contextual cwd) with a
    Git-root placement walk; cold prompt streams normalized to
    started/text_delta/terminal + final text; per-session FIFO
    serialization with cross-session concurrency; permission routing through
    Agent.requestPermission (fails closed); contextual timeout; structured
    teardown that closes handles and aggregates cleanup failures.
• Effection-shaped adaptation: acpx Promise-returning leaves are consumed
  with until; the provider's only Promise-producing adapter is the acpx
  onPermissionRequest callback. The fake runtime returns Promises / an async
  iterable only at the acpx AcpRuntime boundary; its manual-turn gates use
  Effection withResolvers/Futures, not new Promise/async. No as
  assertions; no node:fs in tests.
• Workspace: root deno.json (Deno workspace array + check/test/lint
  tasks), pnpm-lock.yaml, deno.lock, publish workflow (now 8 packages),
  and the release-process-spec.md lockstep list. pnpm-workspace.yaml already
  globs packages/* (no addition); tsconfig.node.json intentionally excludes
  acp (it depends on acpx, stays Deno-typechecked).
• specs/acp-client-spec.md gains a present-tense ACPX provider section.

What is deferred

Named provider selection (<AgentProvider> / registry), permission-policy
components, CLI flags, and the real-subprocess / compiled-binary smoke — the
later general-provider and <TestAgent> phases. DEFAULT_AGENT_NAME is not
re-exported here.

How to verify it

On Deno 2.9.1:

deno task lint
deno check packages/core/mod.ts
deno test --no-check --allow-all packages/core/tests/ packages/durable-streams/tests/
deno task check:jsr
deno task check
deno task test
deno task gen:publish-workflow # no drift (8 packages)
pnpm exec tsc --project tsconfig.node.json --noEmit

Tests are deterministic and fake-runtime-driven (no agent subprocess): Tier
AP (provider: stream shape, timeout, serialization/concurrency, halt/teardown,
doctor caching, permission routing), SQ (serial queues), SK (session
identity), PB (permission bridge), and Tier XA — the acceptance test
installing createAcpxProvider() through #135's rootProvider seam and
exercising the full availability → session → normalized prompt → structured
teardown lifecycle, asserting the runtime handle closes before the
DocumentExecution completion settles. deno task test: 115 passed.

Scope

The ACPX provider library only. The <TestAgent> vocabulary + smoke consume
useAcpxProviderState from this package in the following PR.

---

Targets main. Next: agent/acp-stack-08-test-agent-vocabulary.

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Found 6 redundant comments. Inline suggestions to remove them below.

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PR #136: ✨ feat(acp): ACPX provider package (acp-stack 07)

20 files, +3303 / -114

Scope

🔴 PR has 3417 lines changed. Split into focused PRs.

🟡 3417 lines changed. PRs under 400 receive more thorough review.

🟡 PR mixes config and source changes.

🟡 New abstraction files: packages/acp/src/provider.ts, packages/acp/tests/helpers.ts. Verify 3+ consumers.

🟡 package.json changed without dependency justification.

Structural

✅ No structural bloat detected.

Slop

✅ Slop indicators look low.

Static Analysis

✅ Oxlint found no issues.

Correctness

No extraneous code patterns detected.

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Found 5 redundant comments. Inline suggestions to remove them below.

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