feat(acp): negotiate persistent system prompts

[reinhold-ph]

Why

Buzz currently infers persistent system-prompt support from ACP protocol version or adapter name. Codex ACP reports protocol v1, so Buzz falls back to prepending Base/System/team/core/canvas material to ordinary user messages on every turn.

This adds explicit capability negotiation for the extension implemented by the companion Codex ACP change.

Companion validation PR: Peakhunter/codex-acp#1

Protocol

• Agent advertises _meta.capabilities.persistentSystemPrompt: true in initialize.
• Buzz records only an explicit boolean true; absent, false, and malformed values remain unsupported.
• On session/new, Buzz sends the combined static prompt as string-valued _meta.systemPrompt.
• Buzz then suppresses Base/System/team/core/canvas duplication in ordinary user turns.

Compatibility

• Agents without the capability keep the legacy per-turn framing.
• Claude ACP keeps its existing _meta.systemPrompt: { "append": ... } transport.
• Existing protocol-v2 agents keep the top-level systemPrompt field.
• Goose keeps its post-session custom method and fallback probe.
• Explicit persistent metadata takes precedence over the protocol-v2 field for agents that advertise it.

Validation

RED SHA: 81895074d9fa0e580ddc7f41e037eb65f361a664

GREEN SHA: 282988ea71869e82f05131d88307c4fce3750e49

Local GREEN:

• cargo test -p buzz-acp: 695 unit tests + 9 lifecycle integration tests passed
• End-to-end fake-agent tests compare complete session/new and session/prompt JSON-RPC envelopes, exact prompt-block count/order, and capable/legacy content
• cargo clippy -p buzz-acp --all-targets -- -D warnings
• cargo fmt --all -- --check
• git diff --check

Fork CI at exact SHA 282988ea71869e82f05131d88307c4fce3750e49:

• Feature/source gates GREEN: Rust Lint, Unit Tests, Security, Windows Rust, both Linux cross-compiles, and Desktop umbrella
• Exact-SHA run: https://github.com/Peakhunter/buzz/actions/runs/31280953723
• The aggregate is red only because unchanged desktop/tests/e2e/onboarding.spec.ts:3172 fails its welcome-banner assertion and retry. The same upstream-base failure reproduced across four runs; this branch changes only crates/buzz-acp/src/acp.rs and pool.rs.
• Two independent exact-SHA reviews: APPROVE; no blocking findings.

Base validation/upstream-main-20260808 is exact upstream block/buzz@f029deafae6ad3b63e13c29104f3be76122cb1df.

[cmyk]

Internal validation complete; upstream submission continues at block#5386.