refactor: split the ACP subsystem

[zatevakhin]

Scope

• Replace the ACP monolith with a focused facade and modules for commands, runtime, extensions, replay, notifications, elicitation, JSON-RPC, stdio, and WebSocket transport.
• Isolate stdio, WebSocket, and JSON-RPC boundaries; preserve atomic replay/load buffering; separate notification translation, typed extensions, command routing, and elicitation handling.
• Fix WebSocket teardown ordering so pending JSON-RPC failures are propagated and owned tasks drain before remaining work is aborted. The integrated shutdown_publishes_prompt_failure_before_returning regression test proves PromptFailed is published before run returns.
• Phase 7 reducer work is explicitly excluded.

Review

Independent Linus review identified the WebSocket abort-before-fail defect, approved the lifecycle correction after re-review, and confirmed the final Clippy-only remediation is behavior-neutral and publication-ready.

Validation

• cargo fmt --all -- --check passed
• cargo check --all-targets --all-features passed
• cargo clippy --all-targets --all-features -- -D warnings passed
• cargo build --all-targets --all-features passed
• cargo test --all-targets --all-features passed: 916 passed, 0 failed, 0 ignored
• Focused WebSocket tests passed: 2/2
• git diff --check 9d29638c60d53c533ac365ab2f4667186f105bde..HEAD passed

Environment limitation

External live stdio/WebSocket smoke testing was not run because no verified ACP agent binary or disposable endpoint was available. This is an environment gap, not successful smoke-test evidence.

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