estack-email-writer: implement saved local email guidance from cleanup backup #70

Context

Elliot was preparing to uninstall E-Stack for cleanup reasons. The updater had saved his local email-writer edits in a separate backup folder. He wanted to preserve those edits remotely and track their later implementation in the actual repository. Local deletion was not authorized when this task was created.

The original full skill folder is preserved on the backup branch. Saved SKILL.md.

Changes to review and implement

• Expand email coverage and clarify how the email-writer and cold-message-writer skills work together for first contact.
• Acknowledge obvious weaknesses such as late notice near the opening.
• Allow several small asks in an update email to an already-committed reader.
• Prepare the artifact the reader needs, and ask institutions for the appropriate routing.
• Add guidance for mailing lists, recipient privacy, send records, batching, and personalization.
• Keep audience variants identical except for the factually different section, and preserve deliberately withheld facts across variants.

Acceptance criteria

• Compare the saved 1.2.0 snapshot with production 1.1.1 and review all additions.
• Validate advice about BCC recipient recoverability and provider sending limits against primary sources before incorporating it; the original snapshot is an archive, not a verified implementation.
• Implement approved changes in skills/estack-email-writer/SKILL.md, preserving the original backup snapshot.
• Review representative first-contact, partner-update, mailing-list, and audience-variant drafts against the revised guidance.
• Set the production skill version appropriately and update relevant documentation and changelog entries.
• Complete repository checks and record implementation evidence before closing this issue.

Related: email-writing principles #32. This task tracks the specific preserved local edits and their implementation.