feat(packages/templates,packages/cli): complete draft placeholder support

[zrosenbauer]

Summary

Complete the remaining draft-template placeholder workflow after #152 introduced declared variables, built-in substitution, unfilled-marker reporting, and published-document checks.

Already shipped in #152

• Built-ins: title, slug, date, and filename.
• Declared template variables and repeatable --var id=value input.
• Interactive prompts and an unfilled-marker checklist.
• ciderpress check / build --check failures for markers left in published documents.

Remaining scope

• Add an author built-in sourced from Git configuration with a documented fallback when unavailable.
• Add ciderpress draft --clean to remove template guidance comments from generated documents.
• Define which HTML comments are guidance and preserve comments that are part of the document's intended output.

Acceptance criteria

• {{author}} resolves deterministically or remains visible with an actionable message.
• --clean removes only recognized guidance comments.
• Default draft behavior remains backward compatible.
• Documentation and tests cover Git configuration absence, clean/no-clean output, and preserved comments.

Related

• feat(packages/templates,packages/cli): template fill variables + {{ }} marker convention #152 shipped the core placeholder workflow.