security: establish AgentGuard public security foundation

[ToolOracle]

Summary

Establishes a verifiable public security foundation for AgentGuard.

Security changes

• removes the publicly known fallback signing secret;
• fails closed when AGENTGUARD_SECRET is missing or insecure;
• requires a signing secret of at least 32 bytes;
• replaces hard-coded production runtime paths;
• binds to loopback by default;
• removes internal database paths from health responses;
• sanitizes externally returned tool exceptions;
• removes five duplicated handler implementations.

Repository foundation

• declares runtime and development dependencies;
• adds CI for Python 3.11 and 3.12;
• adds security reporting and configuration guidance;
• adds environment and ignore templates;
• corrects the public tool count from 20 to 17;
• replaces unsupported security and performance claims.

Verification

• 19 local tests passed;
• Python compile check passed;
• all 17 registered tool names are unique;
• historical fallback-secret rejection verified;
• no production deployment is included.

Review status

Draft pending CI and final security review.