feat(acp): compose agent identity into system prompt with fallback

[tlongwell-block]

What & why

Agent identity (operator system prompt + NIP-AE core memory) is composed into a single identity section that rides the existing system-prompt routing, instead of core always being emitted as a standalone [Agent Memory — core] user-message section.

• v2 agents (e.g. buzz-agent, protocol_version >= 2): the composed identity lands in the real system role via session/new.
• v1 / stock ACP agents: the same composed string lands in the user message via the existing format_prompt legacy branch.

No new version fork in routing — the per-version split stays entirely in the existing !has_system_prompt_support gate, which kills double-render by construction.

Composition (core is additive, not exclusive-or)

operator prompt	core fetch	system role includes
present	present	base + operator + core
present	confirmed empty	base + operator
present	failed	base + operator
absent	present	base + core
absent	confirmed empty	base + fallback
absent	failed	base only

Fallback (the repurposed onboarding text → FALLBACK_SYSTEM_PROMPT) fires only when there's no operator prompt and core fetch confirmed-empty. A relay hiccup (Unavailable) never hands an established agent a newbie identity.

Key changes

1. engram_fetch.rs — build_core_section: Option<String> → tri-state fetch_core -> CoreFetch::{Present, ConfirmedEmpty, Unavailable}. The old Option flattened "empty" and "fetch failed" into one None; the enum keeps them distinct — the distinction the fallback rule depends on.
2. pool.rs — two pure helpers (resolve_identity_section, compose_system_prompt) with the truth table in doc comments. Core fetch hoisted above session creation (is_new_session via a cheap map lookup), resolved section cached for the legacy path. SessionState.core_sections → identity_sections (it holds the resolved identity, not raw core).
3. ACP compliance — top-level systemPrompt was a Buzz extension, not ACP. Moved to namespaced _meta: { "buzz.systemPrompt": ... } in both sender (acp.rs) and reader (wire.rs). Reader is crash-proof on malformed _meta and now ignores the bare top-level key; regression tests pin that.

Testing

• Full suites green: 332 buzz-acp + 98 buzz-agent + integration, 0 failures. clippy clean.
• Guardrail tests: test_format_prompt_modern_agent_suppresses_base_and_system (modern → no duplicate [Agent Memory] in user message) and test_format_prompt_legacy_agent_emits_base_and_system (legacy → composed identity via [System], delivery-neutral).
• Pre-push hook ran the rust suite green.

Reviewer: please also run locally per TESTING.md.