feat(llm-mcp): Mode B Spec 520 prepare/cache scaffold

[enricopiovesan]

Summary

Scaffolds Mode B under apps/llm-mcp-reference/mode-b/: Spec 520 prepare/verified-cache contract, prepare-cache.sh, fail-closed serve.sh, and mcp.json.example. Updates plan/README/AGENTS and extends secondary smoke. Does not claim a live offline MCP host — that waits on Traverse #865.

Governing Spec

• docs/llm-reference-apps-plan.md (Mode B)
• Traverse Spec 520 / #860 (cache library shipped)
• Project 2 ticket llm-mcp-embedded-host

Project Item

• Ticket ID: llm-mcp-embedded-host
• Item: PVTI_lADOEbiBt84BbzAzzg0epTo

Definition of Done

• Mode B docs + launcher + example MCP config
• No App-Refs materialize rewrite in Mode B path
• Smoke asserts scaffold + fail-closed serve
• Honest status: live Mode B host → Traverse #865
• Live offline MCP execute from prepared cache (upstream)

Validation

• bash scripts/ci/llm_mcp_reference_smoke.sh
• bash scripts/ci/repository_checks.sh

Checklist

Traceability

• Project 2 ticket updated (Agent + Status); Ticket ID cited above
• Updated AGENTS.md / plan docs for shipping status
• Remaining upstream blocker called out (Traverse #865)

Production baseline (platform / runtime-client PRs)

• Embedded mode — Mode B documents Spec 520 cache; Mode A remains runnable default
• Digest pin / sync — N/A (no materialize rewrite; cache host-owned)
• SDK test doubles — N/A (scaffold/docs + smoke)
• Docs touchpoints — README + plan updated
• CI commands — listed under Validation

Architecture

• No business logic added to UI/façade layer
• No private Traverse internals imported
• CI gates pass locally where applicable

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