Centralize timeout budget handling

[ryanfowler]

Summary

• Move shared timeout parsing, Go-style duration formatting, remaining-budget calculation, and timeout error formatting into src/duration.rs
• Reuse TimeoutBudget across HTTP client setup, WebSocket dialing, DNS inspection, and TLS inspection instead of duplicating elapsed-time logic
• Update related duration validation call sites and document the new ownership in AGENTS.md

Testing

• cargo fmt
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --all-features