Centralize config option handling in config parsing

[ryanfowler]

Summary

• Introduce a typed config option descriptor table in src/config/mod.rs
• Drive parsing, overlaying, CLI source tracking, and config application from the same metadata
• Add parity tests to catch drift between config keys, CLI flags, and documented options
• Update AGENTS.md with the new maintenance rule for config-backed options

Testing

• cargo test --all-features
• cargo test --all-features --test cli --test formatting --test grpc --test http --test network --test terminal --test update --test websocket -- --test-threads=1