[skip-release] Make native build timeout configurable

[libops-agent]

The shared publisher currently hard-codes a 30-minute native build timeout. That is too short for deterministic cold builds such as the API Terraform runner, which compiles exact provider releases for both architectures before publication.

This change:

• adds a numeric build-timeout-minutes reusable-workflow input with a 30-minute default;
• restricts accepted values to 5 through 120 minutes;
• applies the input only to each native build job; and
• adds a Go contract test for the default, expression, and validation boundary.

Existing callers keep the same bounded default. Callers with reviewed cold-build requirements can opt into a larger timeout without copying the publisher.

Validated locally with actionlint, go test ./ci/github, and git diff --check.