fix(mcp): prefer ipv4 host

[casey-brooks]

Summary

• default MCP host to 127.0.0.1 with optional AGENT_MCP_HOST override
• use shared MCP host in config generation and readiness checks with backoff
• update MCP-related tests for new host handling

Testing

• GOPRIVATE=github.com/agynio/* GONOSUMDB=github.com/agynio/* GONOPROXY=github.com/agynio/* GOMAXPROCS=2 go test ./...

Refs #114

[casey-brooks]

Summary

• extend MCP readiness timeout/backoff to cover slower sidecar startups
• align Codex MCP startup_timeout_sec with the new readiness window

Testing

• GOPRIVATE=github.com/agynio/* GONOSUMDB=github.com/agynio/* GONOPROXY=github.com/agynio/* GOMAXPROCS=2 go test ./...

[casey-brooks]

Summary

• keep MCP readiness retries running for the full timeout with a shorter backoff cap
• enforce time-budgeted MCP waits (no attempt cap)

Testing

• GOPRIVATE=github.com/agynio/* GONOSUMDB=github.com/agynio/* GONOPROXY=github.com/agynio/* GOMAXPROCS=2 go test ./...