fix: honor Nitrogen config path

[huytdps13400]

Summary

• pass the parsed --config value into Nitrogen before generation starts
• centralize file-backed singleton initialization in NitroConfig.load() while preserving ./nitro.json as the default
• add a black-box CLI regression and run Nitrogen CLI tests in CI

Breaking changes

None. Existing invocations still load ./nitro.json by default.

Verification

• regression test failed before the fix: the CLI logged Loading nitro.json and reported that ./nitro.json did not exist despite a valid --config path
• bun --cwd packages/nitrogen test (1/1)
• bun --cwd packages/nitrogen typecheck
• bun --cwd packages/nitrogen lint-ci
• bun run build
• bun typecheck
• bun specs, with no generated diff
• C++ and Swift format scripts passed through the Xcode toolchain
• full lint-all reached Kotlin lint but the local machine does not have the ktlint executable; no Kotlin/native files are changed and CI will run that lane on the configured runner

AI assistance

Codex using GPT-5.6 Sol assisted with reproduction, implementation, tests, and review.

Fixes #1554

[vercel]

@huytdps13400 is attempting to deploy a commit to the Margelo Team on Vercel.

A member of the Team first needs to authorize it.

[mrousavy]

that's not safe and just prone to races.