feat(acp): support mid-turn steering / 支持 ACP 回合中引导

[gcoder1991]

Summary

• Expose Reasonix's existing mid-turn steer queue through the ACP v1 vendor extension _reasonix.io/session/steer.
• Advertise the extension through agentCapabilities._meta["reasonix.io"].sessionSteer, so clients do not have to guess whether the method exists.
• Accept the same {sessionId, prompt} content-block shape as session/prompt, but only while a prompt is active; idle sessions return InvalidRequest instead of silently starting a new turn.
• Keep existing CLI/Desktop/Bot fallback behavior unchanged by splitting strict TrySteer acceptance from the existing Steer fallback.
• Replay persisted steers as the user's raw text instead of leaking Reasonix's internal steer wrapper.
• Add dedicated bilingual ACP integration guides and surface them from both READMEs, the main guides, and the project website.

This is intentionally a Reasonix ACP v1 extension, not an implementation of the still-unreleased ACP v2 session/inject proposal.

Refs #3470

ACP v2 context: agentclientprotocol/agent-client-protocol#1261

Documentation

• docs/ACP.md and docs/ACP.zh-CN.md consolidate startup, capability negotiation, session lifecycle, independent controls, host-supplied MCP transports, permissions, steering errors, compatibility, cache behavior, and a client checklist.
• Both READMEs and the full English/Chinese guides now link to the dedicated ACP section instead of duplicating a partial protocol description.
• The bilingual website documents ACP at /docs/#acp, links the complete source guides, and exposes reasonix acp from the homepage.

Capability discovery

The initialize response includes:

{
  "agentCapabilities": {
    "_meta": {
      "reasonix.io": {
        "sessionSteer": {
          "method": "_reasonix.io/session/steer"
        }
      }
    }
  }
}

Wire shape

{
  "jsonrpc": "2.0",
  "id": 2,
  "method": "_reasonix.io/session/steer",
  "params": {
    "sessionId": "session-id",
    "prompt": [{"type": "text", "text": "use plan B"}]
  }
}

Successful acceptance returns {}. Unknown sessions and empty prompts use the existing ACP validation errors; sessions without an active prompt return -32600 InvalidRequest. The unnamespaced session/steer method is intentionally not registered because ACP v1 reserves non-underscore method names for the core protocol.

Backward compatibility

Surface	Existing behavior	Result
Existing ACP v1 methods	Unchanged	Compatible
agentCapabilities._meta	Additive and ignored by clients that do not understand it	Compatible
Persisted transcripts	No persisted schema change; replay hides the internal steer wrapper	Compatible
CLI/Desktop/Bot steer	Existing idle and turn-exit fallback remains unchanged	Compatible

Provider-visible prompts, tool schemas, and cache-stable prefixes are unchanged apart from the user-requested steer message entering normal conversation history.

Verification

• go test ./internal/acp -run 'TestServe(Lifecycle|SteerInjectsIntoActivePrompt)$' -count=20
• go test ./internal/acp ./internal/control ./internal/agent
• go test -race ./internal/acp ./internal/control ./internal/agent
• go test ./internal/acp ./internal/cli ./internal/control ./internal/agent
• go vet ./...
• env -u DEEPSEEK_API_KEY go test ./...
• cd site && npm test
• cd site && npm run build
• Local Markdown link and bilingual ACP protocol-token parity checks
• Chrome DevTools checks for English/Chinese ACP deep links at desktop and 390px mobile widths, with no console errors, failed resources, or horizontal overflow
• git diff --check
• Synthetic merge with the latest origin/main-v2, followed by go test ./internal/acp ./internal/control ./internal/agent

Focused coverage verifies capability discovery, rejection of the reserved unnamespaced route, active-turn injection, idle rejection, model visibility at the next safe loop boundary, and replay without the internal wrapper.

Cache impact

Cache-impact: none - ACP transport and documentation/site only; no provider-visible system prompt, tool schema, or cache-stable input changes.
Cache-guard: go test ./... and focused ACP steer, capability, and replay regression tests cover the changed surface.
System-prompt-review: N/A