feat(cli): add lint command

[nucleuskit]

Summary

• add the nucleus lint subcommand with human and JSON output aligned with validate
• wire lint into the root CLI and add command/output tests
• pass strict mode directly into contract/lint's single Run(dir, strict) entry point
• update the hello-http example so strict lint passes
• update the contract submodule pointer for the lint API refactor

Dependency

• Depends on refactor(lint): collapse strict dispatch contract#3 for the contract/lint strict-mode API refactor.

Verification

• rtk go test ./...
• rtk go test ./... -race -count=1
• rtk go run ./cmd/nucleus lint --dir example/hello-http --strict --json
• rtk go run ./cmd/nucleus validate --dir example/hello-http
• cd contract && rtk go test ./...

Residual risk

• Root-level validate --dir . and lint --dir . still fail because the repository root is not a service directory and has no nucleus.yaml.
• verify is not registered yet in the CLI.