Base pack: warn when target project has no license declared

[Fieldnote-Echo]

Context

nboot's own source files have SPDX headers (MIT) and a pre-commit hook enforcing them. But when nboot renders packs for other projects, it should respect their license strategy — not assume one.

Desired behavior

• Warn when spec.license is not declared — emit a CLI warning during render/apply
• Never inject a license choice into a target project unprompted
• When spec.license IS declared, generated files can include appropriate SPDX headers
• The pre-commit hook for SPDX enforcement should be conditional on spec.license being present

Scope

Pack-level concern (base pack templates + CLI warning), not an engine change.

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