Implement strict Package Template References

[zack-nova]

Closes #140

Summary

• render strict {{ vars.<name> }} Package Template References across all valid text files
• validate template references against manifest declarations for apply/reapply/replay/preview flows
• update reverse variableization and backfill output to emit strict references
• keep binary/invalid UTF-8 files untouched and preserve GitHub Actions ${{ ... }} expressions

Dev Brief Mapping

• Parser/renderer rejects malformed references, unsupported namespaces, undeclared variables, and unresolved strict bindings when required.
• Rendering is no longer Markdown-only; legacy $name references are not rendered.
• Save/backfill reverse replacement emits {{ vars.<name> }}.

Validation

• go test ./cmd/orbit/cli/template -count=1
• go test ./cmd/orbit/cli -count=1
• mise run fix
• mise run ci