docs: add mailon-backup integration plan

[ShinHur]

Summary

Adds integration-plan.md — the design and validation record for connecting a Windows-side mailon-backup CLI to the Autophagy site mail account through the existing site_mail_backend.py contract, with zero changes to Autophagy code (the connection is env var + config JSON + an external adapter outside this repo).

Contents

• Analysis of skills/mail, the site-mail backend contract, and the 5-op (list/get/status/resolve/send) stdin/stdout JSON protocol
• Adapter placement and a no-modification integration structure
• Validation log: reads e2e (real store), live resolve (counts only), send --dry-run (no send), CDP lifecycle, and Windows-env reconstruction

Sanitization

Host-specific identifiers replaced with placeholders. Both hygiene gates pass:

• repo_scan --profile public-generic → SCAN-CLEAN findings=0
• repo_scan --profile docs-claims → SCAN-CLEAN findings=0

Note: operator-facing integration plan; if out of scope for the public tree, feel free to close.

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