docs: add the agent-mesh Field Guide

[hartsock]

Summary

Adds docs/field_guide.md — a single operator-facing explainer with eight
mermaid diagrams (GitHub renders them natively):

1. The four-layer crate stack and its consumers (newt-mesh, newt-web, amesh, amesh mcp)
2. The trust chain: GitHub SSH key → UserKey → AgentKey → SignedEnvelope
3. The wire sequence: mDNS hints → auto-team QUIC handshake → per-envelope verify
4. The authority-provenance CID DAG (real, 0.6.4) vs the agent-mesh-store
   per-writer merkle logs (ADR only, no code) — explicitly badged TODAY vs PLANNED
5. An honest comparison with tmux-message-bus (the durable-inbox gap)
6. newt-agent's two live integrations: inference dispatch and docking (bus: handle_requests_with_context — expose the verified caller principal #75)
7. wyvern-agent's status (no mesh code yet) and its two integration paths
8. A zero-to-two-agents runbook

The known auto-team handshake residual is stated plainly rather than papered
over ("a filter, not a proof").

Test plan

• Docs-only change; no code touched.
• Full pre-push gate ran on push (fmt, clippy -D warnings, workspace tests, cov-ci ≥75%): green.
• Mermaid blocks use plain flowchart/sequenceDiagram syntax that GitHub's renderer supports.

Risk: low (documentation only).

🤖 Generated with Claude Code

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