fix(ci): improve gateway CI path triggers

[chaodu-agent]

Description

The gateway CI job added in #769 has a few path trigger gaps that could cause missed or unnecessary CI runs.

1. Missing gateway/Cargo.lock — If the gateway maintains its own lockfile, dependency-only updates (e.g. Dependabot PRs) won't trigger the gateway CI job, allowing broken dependency upgrades to pass silently.

2. Root Cargo.toml/Cargo.lock triggers both jobs — Changes to the root lockfile trigger the gateway job even when no gateway code changed. Minor inefficiency but wasteful on runner minutes.

Related: #769

Steps to Reproduce

1. Submit a PR that only modifies gateway/Cargo.lock
2. Observe that the gateway CI job does not run

Expected Behavior

• gateway/Cargo.lock in the paths list so dependency-only gateway PRs get tested
• Optionally scope root Cargo.toml/Cargo.lock to only trigger the core job (or accept the current behavior as acceptable overhead)