feat(buzz-acp): route goose systemPrompt via ACP+ _meta extension

[wpfleger96]

Reference implementation of RFD #1237, which is not yet merged. The RFD proposes systemPrompt as a first-class session/new field in ACP. Until it lands, buzz passes systemPrompt to goose agents via _meta["systemPrompt"] — the ACP extensibility mechanism — matching the goose-side implementation in aaif-goose/goose#9971.

For goose agents, systemPrompt must be passed via _meta["systemPrompt"] rather than as a first-class session/new field, because goose reads it from _meta until RFD #1237 lands. Buzz-agent native agents continue to use the top-level systemPrompt field unchanged. When the RFD merges, both this PR and the goose companion can be updated to use the first-class field directly.

Changes

• crates/buzz-acp/src/acp.rs — adds session_new_full_goose, which puts systemPrompt inside _meta instead of at the top level. Two new unit tests verify the goose path: session_new_full_goose_includes_system_prompt_in_meta_when_some and session_new_full_goose_omits_meta_when_none. Existing session_new_full and its tests are unchanged.
• crates/buzz-acp/src/config.rs — promotes normalize_agent_command_identity to pub(crate) so pool.rs can use it for goose detection.
• crates/buzz-acp/src/pool.rs — adds agent_command: String to PromptContext; create_session_and_apply_model dispatches to session_new_full_goose when normalize_agent_command_identity(&ctx.agent_command) == "goose", otherwise falls through to session_new_full.
• crates/buzz-acp/src/lib.rs — populates agent_command in the PromptContext constructor from config.agent_command.

Related

• agentclientprotocol/agent-client-protocol#1237 — RFD proposing systemPrompt as a first-class ACP field (not yet merged)
• aaif-goose/goose#9971 — goose-side companion: reads systemPrompt from _meta and applies it to the session