docs(security): define exact error boundary

[jmgilman]

Summary

• document where trusted diagnostic causes exist and where root and MCP projections remove them
• distinguish direct authz/rego.Authorize detail, root sentinels, fixed MCP texts, and pre-resolution SDK schema errors
• add non-disclosing recovery actions and state that one policy result cannot reveal policy defaults or completeness
• align error projection comments without changing behavior

Verification

• focused direct Rego, root projection, MCP projection, panic-sanitization, and secure-loop tests passed
• campaign response-only corpus: 17 canary-witness MCP responses had zero canary matches; all 5 fixture positive controls contained the canary
• campaign M08 response corpus: 9 MCP error payloads had zero matches across 13 source, argument, credential, policy, handler, panic, and internal-detail terms
• mise exec -- moon run root:check (all eight tasks passed)
• LSP diagnostics on server.go and mcpserver/server.go: no issues