feat(unstable): add client system prompt schema

[benbrandt]

Implements the schema for the Client-Provided System Prompt RFD (#1237) in ACP v1 and v2, behind a new unstable_session_system_prompt feature that is part of the unstable umbrella. The stable schemas are unchanged.

Schema

• Capability: sessionCapabilities.systemPrompt in v1, session.systemPrompt in v2. {} advertises append mode; { "mode": { "override": {} } } also advertises override mode.
  • Types: SessionSystemPromptCapabilities, SessionSystemPromptModeCapabilities, SessionSystemPromptOverrideCapabilities.
• Request field: optional systemPrompt: { mode, content, _meta? } (SystemPrompt, SystemPromptMode) on session/new, session/resume, and session/fork, plus session/load in v1. Each request gets a system_prompt(...) builder.
  • mode is required on the wire, so every request states its mode.
  • In Rust, SystemPromptMode defaults to Append, SystemPrompt::new takes only the content, and .mode(...) selects override. Serialization always writes mode.

Design notes

• Strict request field, lenient capabilities. Capabilities use DefaultOnError, so a malformed capability means "not advertised". systemPrompt has no fallback: the agent acts on it, so a malformed value fails the request with -32602 instead of creating a session without the prompt. This covers a missing or null mode, a missing content, a malformed content block. null is equivalent to omitting the field.
• mode is extensible in both versions. SystemPromptMode has the standard Other(String) fallback under the enum variant extension rule, as for v2 MCP server transports: unknown modes parse so proxies can forward them, and agents MUST return an error for modes they don't support rather than treating them as append or override.
• v2 unknown content blocks. v2's ContentBlock keeps unknown block types as ContentBlock::Other. The field docs tell agents they MUST reject block types they don't support.
• override is a Rust keyword, so the capability field and builder are override_ with #[serde(rename = "override")], following the existing type_ fields.
• SystemPrompt derives Eq, so the lifecycle requests keep their stable Eq derive. This relies on content being Eq since feat(rust): implement Eq for content blocks #2352.

Docs

• Draft v1 and v2 initialization guides: the new capability.
• Draft v1 and v2 session-setup guides: a new "System Prompt (unstable)" section covering modes, lifecycle resend semantics, errors, and capability gating.
• Regenerated schema.unstable.json and draft schema.mdx for both versions.
• RFD: implementation note and revision history, plus one spec change: mode is now extensible rather than closed.

Testing

• npm run check (clippy in three configurations, format, spellcheck, all-features tests)
• cargo test -p agent-client-protocol-schema with no features, with unstable_session_system_prompt, and with it plus unstable_protocol_v2
• New tests in v1 and v2:
  • capability round-trips, including the nested override, and fallback on malformed capabilities (a malformed mode capability means append only)
  • request round-trips on every lifecycle request, and override round-trips
  • null equivalent to omitted for systemPrompt, checked on every lifecycle request through a shared helper
  • rejection of malformed values on every lifecycle request: string prompt, missing or null mode, missing content, a non-string mode, non-array content, and a malformed text block
  • both versions preserve unknown modes as SystemPromptMode::Other; v1 rejects unknown content block types, while v2 preserves them as ContentBlock::Other
• npm run generate is idempotent; mint broken-links passes
• A scratch merge with current main regenerates identically and passes the system prompt tests

Not included

• SDK example agents and clients live in the SDK repositories and aren't updated here.
• The feature stays unstable until the RFD moves to Preview.