fix(desktop): render goose's system prompt in the activity log

[sumit-m]

Problem

Goose agents never show a System prompt card in the agent activity log. Every other runtime does.

The transcript builder turns a session/new request into that card by reading params.systemPrompt. Goose does not receive its system prompt that way — buzz-acp sends it as a separate request immediately after session/new:

// crates/buzz-acp/src/acp.rs — session_set_goose_system_prompt
self.send_request(
    "_goose/unstable/session/system-prompt/set",
    json!({ "sessionId": session_id, "mode": "append", "key": "buzz", "text": text }),
)

agentSessionTranscript.ts has no branch for that method, so the request is observed but never rendered. The result is that a goose agent's base prompt, persona, team instructions, core memory and canvas are invisible in the UI — there is no way to see what the agent was actually told.

Fix

Read params.text from that request and upsert the same "System prompt" metadata item the session/new path already produces.

The request lands before the session's first turn, so reusing the existing acpSource: "session/new" places the card in the same standalone slot at the head of the session run — no changes to grouping or presentation.

Notes

• Purely additive: one new else if branch. No existing path changes, and nothing renders differently for runtimes that already worked.
• Goose falls back to user-message framing when the custom method is unsupported (goose_system_prompt_supported == Some(false)). That path is untouched — [Base] continues to appear in Prompt context as before.
• The Rust side already pins the request shape (goose_system_prompt_request_uses_append_contract asserts method, mode, key and text on the bytes written), and write_ndjson observes every outbound message, so the frame this branch consumes is guaranteed to be emitted.

Test

One new test asserting a goose system-prompt request produces exactly one card, with the same ["Base", "System"] sections, outside any turn bucket.

pnpm test 3886 passing, pnpm typecheck clean, biome check clean.

Not verified against a live goose agent — I have no goose model configured. The chain is covered by tests on both sides of the wire.