feat(core,a2a): Focus strategy auto-consolidation and MMA2A AgentCard capabilities

[bug-ops]

Summary

• research(core): Focus agent — model-controlled active context compression with Knowledge block (arXiv:2601.07190) #3313 (Focus): wires the previously-declared CompressionStrategy::Focus into the proactive compaction pipeline. When active, the agent identifies the oldest low-relevance message window via TF-Jaccard scoring and extracts key facts into an AutoConsolidated KnowledgeBlock using focus_scorer_provider. AtomicBool guard is now RAII (CompressionGuard) — safe under future cancellation and panic.
• research(a2a): MMA2A — multimodal-native routing in A2A networks (arXiv:2604.12213) #3326 (MMA2A): adds images/audio/files capability fields to AgentCard with full backward-compatible #[serde(default)] and fluent builder helpers. Bootstrap wiring deferred to feat(a2a): wire AgentCard modality capabilities at A2A bootstrap #3378.

Changes

zeph-core:

• run_focus_auto_consolidation — TF-Jaccard scoring + LLM extraction with 20s timeout
• run_focus_auto_consolidation_pass — Focus branch in maybe_proactive_compress
• CompressionGuard RAII struct — releases compressing AtomicBool on drop
• Skips consolidation when no User message exists or focus_scorer_provider is misconfigured

zeph-config:

• FocusConfig.auto_consolidate_min_window (default 6)
• migrate-config step 31 via toml_edit

zeph-context:

• should_proactively_compress handles CompressionStrategy::Focus

zeph-a2a:

• AgentCapabilities.{images,audio,files} fields
• AgentCardBuilder.{images,audio,files}() helpers

Test Plan

• cargo nextest run -p zeph-config -p zeph-a2a --lib --bins — 409 tests pass
• cargo nextest run -p zeph-core --lib --bins — 1488 tests pass
• cargo nextest run -p zeph-context -E 'test(focus)' --lib — 3 tests pass
• cargo +nightly fmt --check — clean
• cargo clippy -p zeph-core -p zeph-config -p zeph-context -p zeph-a2a -- -D warnings — clean
• Live test with [memory.compression] strategy = "focus" config — playbook: .local/testing/playbooks/focus-auto-consolidation.md
• Live test for AgentCard modality fields — playbook: .local/testing/playbooks/mma2a-capabilities.md

Follow-up Issues Filed

• feat(a2a): wire AgentCard modality capabilities at A2A bootstrap #3378 — wire AgentCard modality at A2A bootstrap (deferred from research(a2a): MMA2A — multimodal-native routing in A2A networks (arXiv:2604.12213) #3326)
• perf(core): offload score_blocks_mig O(K2) pairwise scoring to spawn_blocking #3386 — offload score_blocks_mig O(K²) to spawn_blocking
• fix(core): validate auto_consolidate_min_window rejects zero #3387 — validate auto_consolidate_min_window rejects zero

Closes #3313
Closes #3326