feat: surface base + persona system prompts in observer feed

[wpfleger96]

The agent observer "Prompt context" panel showed only the per-turn sections (core memory, the triggering event) and never the base or persona system prompt — Will's screenshot showed "2 sections" with both system prompts absent. The data was already on the wire: session/new emits an acp_write observer frame carrying params.systemPrompt. The desktop transcript builder simply never parsed the session/new frame.

This surfaces the base + persona prompts as one collapsible "System prompt" metadata item with two labeled sub-sections (Base, System) — both are parts of the single systemPrompt field sent to the LLM, so they're grouped under one item.

Harness (crates/buzz-acp/src/pool.rs)

The modern path (protocol_version >= 2) joined base + persona with a bare \n\n, erasing the boundary. The new framed_system_prompt helper frames each present prompt with its own header, reusing queue::base_section as the single source of truth for [Base] framing:

• both present -> [Base]\n{base}\n\n[System]\n{persona}
• base only -> [Base]\n{base}
• persona only -> [System]\n{persona} (a bare persona would be mislabeled Base downstream)
• neither -> None

The legacy path (protocol_version < 2) is unchanged — it already frames [Base]/[System] in the per-turn message.

Desktop (agentSessionTranscript.ts, agentSessionTranscriptHelpers.ts)

A new method === "session/new" arm in the acp_write branch parses params.systemPrompt and emits the item via the existing upsertMetadata helper. The split is deterministic (parseSystemPromptSections) rather than the generic section parser:

• Partitions on the first \n[System]\n boundary; each labeled body is read literally.
• Embedded [...] lines inside a body never start a new section, so a persona containing a bracketed line can't inflate the section count.
• An oversize-elided [System] header degrades to a single correctly-labeled Base section (truncated body) — never a dropped label, never a lying count.

The item is keyed system-prompt:${ch} (per channel-session): the session/new frame predates session creation and carries no session id, and the frame fires once per channel-session lifetime, so a re-created session correctly replaces the prior item. No change to PromptSection / TranscriptItem types.

Tests

• framed_system_prompt: both-present, base-only, persona-only, neither.
• parseSystemPromptSections: both-present (2 sections), base-only (1 Base), persona-only (1 System), embedded bracket lines stay literal, elided [System] header degrades to labeled Base, empty input -> no sections.