Fixing Provider Registry Mutations and Sandbox Permissions in git-with-intent

The CustomProviderRegistry in git-with-intent had a critical bug where registering custom LLM providers mutated module-level global constants (PROVIDER_CAPABILITIES and PROVIDER_COSTS), causing cross-tenant pollution, test isolation failures, and race conditions. The fix moved capabilities and cost metadata into per-instance storage with a lookup chain that shadows built-ins, eliminating all shared mutable state. The same release also hardened the SandboxedAgent with deny-by-default network access and stricter permission enforcement.