json output + stable exit codes (M2M / LLM contract)

- -json on info, list, show, cert (+ -chain): emit structured JSON ready
  for jq / agent consumption
- exit codes are now stable + documented: 0/1/2/3/4/5/130 with distinct
  meanings for network vs integrity vs signature failures
- help text enumerates the contract
- readme: machine-usage section explains both

MCP stdio server (for Claude Code / Cursor etc.) planned, not wired yet

README.md

@@ -92,6 +92,36 @@ STM (Stake-based Threshold Multi-signature) aggregate proof over BLS12-381.
   downloaded — 16836 entries for preprod as of epoch 284).
 - Full immutables loop for the `download -immutables` path.
 
+## Machine / LLM usage
+
+Every query command accepts `-json` for structured output:
+
+```
+mithril-go list   -network mainnet -json      # snapshot array
+mithril-go show   -network mainnet latest -json
+mithril-go cert   -network mainnet head -json
+mithril-go cert   -network mainnet head -chain -json
+mithril-go info   -network mainnet -json
+```
+
+Exit codes are stable:
+
+| Code | Meaning |
+|---|---|
+| 0 | success |
+| 1 | generic error |
+| 2 | usage error |
+| 3 | network / aggregator error |
+| 4 | integrity failure (SHA, tar-slip, truncated archive) |
+| 5 | signature verification failure (genesis or STM) |
+| 130 | canceled (SIGINT) |
+
+These are the contract — existing codes won't renumber.
+
+Planned: `mithril-go mcp` stdio server (Model Context Protocol) so MCP-native
+agents (Claude Code, Cursor, etc.) can discover + call commands without
+shelling out. Not yet implemented.
+
 ## Dependencies
 
 - `github.com/klauspost/compress/zstd` — pure Go zstd decoder