ci(acp): add a command-neutral acceptance probe

[dspury]

Summary

Adds a command-neutral acceptance probe for ACP executables. Callers provide the executable and repeatable arguments explicitly; Buzz verifies that the command completes ACP initialize and session/new through buzz-acp models within a bounded timeout.

The contract depends only on the public ACP process boundary. It does not assume a particular runtime, host, relay, private key, deployment topology, or lifecycle manager.

What changed

• Adds scripts/agent-harness-acceptance.sh with required --agent-command, repeatable --agent-arg, and configurable --timeout options.
• Extends buzz-acp models with additive --agent-arg and --timeout options while preserving the existing positional argument behavior.
• Preserves argument boundaries exactly, including spaces and commas.
• Adds scripts/test-agent-harness-acceptance.sh, backed by a synthetic ACP command and fake buzz-acp, so CI validates success and failure behavior without installing any external runtime.
• Wires the synthetic contract test into the existing GitHub Actions quick script checks.
• Documents the public invocation in crates/buzz-acp/README.md.

The earlier unrelated ARCHITECTURE.md protocol-limit correction and named runtime presets are intentionally not part of this revision.

Why

An external ACP implementation should be testable against Buzz without adding runtime-specific discovery, installation, credentials, machine assumptions, or deployment policy to this repository. This probe gives adapter authors a stable executable contract while keeping those concerns with the caller.

Validation

• bash -n scripts/agent-harness-acceptance.sh scripts/test-agent-harness-acceptance.sh
• scripts/test-agent-harness-acceptance.sh
• cargo test -p buzz-acp
• cargo clippy -p buzz-acp --all-targets -- -D warnings
• Real synthetic ACP handshake through the built buzz-acp, including arguments containing spaces and commas
• just ci

The full just ci gate passed on 2026-08-13, including the current Rust, desktop/Tauri, web, and mobile checks, tests, and builds.

Rebuilt as one signed commit on upstream/main at a96af8952.