feat(v2/config): add ResolvePath for config file discovery

Adds an opt-in config.ResolvePath() that finds a service's config file the same way Rust labkit_config::resolve_path() does: the Fairway-mounted $CONFIG_DIR/config.yaml first, then $CONFIG_PATH, then config/local.yaml for local runs. Today each Go service writes this lookup itself, so Go and Rust services can look in different places. Loader is unchanged and still never reads environment variables.

This lets the Go Lab Bench chassis find appconfig exactly like the Rust one, which unblocks gitlab-org/lab-bench/bench#106 without interim flag plumbing in bench.

Closes #174 (closed)

🤖 Generated with Claude Code