feat(workflows): add project hooks

[DonPrus]

Summary

• add configurable target matrices to CI, nightly, and release reusable workflows
• add optional Node setup plus project hook commands for generated assets and other repo-specific prep
• add a single-target CI E2E command hook so callers can keep behavior checks inside the shared matrix
• add optional release source archive support with caller-controlled prepare command and tar excludes
• document a NullHub-style generated-assets setup

Validation

• ruby -e 'require "yaml"; Dir[".github/workflows/*.yml"].each { |f| YAML.load_file(f); puts "ok #{f}" }'
• ruby -e 'require "yaml"; require "json"; [".github/workflows/zig-ci.yml", ".github/workflows/zig-release.yml", ".github/workflows/zig-nightly.yml"].each { |f| y=YAML.load_file(f); inputs=y.fetch(true).fetch("workflow_call").fetch("inputs"); inputs.each { |k,v| JSON.parse(v["default"]) if k == "targets_json" }; puts "json ok #{f}" }'
• git diff --check