[wpfleger96]

Stacked on #1227 (shared ACP client extraction). Stack: #1227 → this PR → generic ACP plugin runtime (Hayt).

Lets the shared ACP client drive a harness that has none of Goose's extensions, which the generic ACP plugin needs, and moves the Buzz-tool server pool into the shared client so every ACP plugin shares one.

Harnesses without hooks (every AcpHooks hook is now optional)

• System prompt: a systemPrompt hook wins; otherwise ACP's systemPrompt (mode: "append") goes on session/new when sessionCapabilities.systemPrompt is advertised; otherwise the instructions lead the session's first prompt, once.
• Steering: without a steering hook, a message sent mid-turn runs as the next turn.
• Model: without a select hook, the harness's own pick stands. Without newSessionMeta, no _meta is sent.
• session/close is sent only when sessionCapabilities.close is advertised, or when hooks set closeSession: "always" (Goose does, keeping its behavior).
• mcpCapabilities.http must be exactly true, else the harness is refused.

Startup and shutdown

• AcpProcess.start takes args (passed to spawn unchanged) and a signal; aborting kills the process and rejects with the signal's reason.
• BUZZ_PRIVATE_KEY, BUZZ_AUTH_TAG and BUZZ_RELAY_URL are removed after the hooks' and launch's env (PROTECTED_ENV).
• Exit 126/127 before initialize answers maps to hooks.missing; stderr written before startup finishes is kept for the error.
• AcpProcess.stop(timeout) stops the harness and resolves once it exits or the time limit passes.

Shared server pool

• SHARED_POOL (48 servers) lives in agent-sessions.ts; AgentSessions uses it unless given a pool. Goose's runtime no longer makes its own, so Goose and generic sessions evict each other's least recently used idle session.

Goose's test files and goose-testing.ts are unchanged.

[wpfleger96]

🤖 Folded into #1227.