Validate entries against omit_pr and omit_author

[zedoraps]

🔍 Problem

• omit_pr: true / omit_author: true only guard the add command.
• Entries written directly to disk (e.g. by agents) bypass the options entirely, so prs/authors metadata slips into projects that forbid it.
• Concrete fallout: platform-internal sets omit_pr: true, yet published entries carry PR numbers that render as 404 links to the public repo.

🛠️ Solution

• validate now errors on entries carrying prs metadata when the config sets omit_pr: true, and authors metadata when it sets omit_author: true.
• Applies to unreleased and released entries, and to nested modules via their own configs.

💬 Review

• Projects that enabled the options after entries were already published will start failing validation until the offending fields are scrubbed — intentional, since those fields violate the config's stated policy.
• Composes with the new require_pr option from Require PR metadata for changelog entries #34, which already skips its check when omit_pr is set.

_{📚 Docs PR: tenzir/content#80
📎 Related: #33}