Add project vision document

VISION.md

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+# MailPlus Intelligence Vision
+
+Version: 0.1
+
+MailPlus Intelligence is a privacy-aware analysis layer for Synology MailPlus data, focused on useful local intelligence rather than cloud-dependent mailbox automation.
+
+It should help the operator understand mail patterns, threads, references, and governance-relevant signals while keeping fixture-bound tests and private data boundaries explicit.
+
+## Who It Serves
+
+- The operator managing Synology-hosted mail.
+- Agents repairing parsing, threading, and governance workflows.
+- Future users who need local mail intelligence without handing over mailbox contents.
+
+## Product Principles
+
+- Fixture-bound validation is the default proof path.
+- Malformed references and edge cases should become tests.
+- Public readiness requires careful separation from live private mail.
+- Threading behavior must be explainable and inspectable.
+- Governance docs should match the code's actual privacy boundary.
+
+## Near-Term Direction
+
+- Harden threading and malformed-reference handling.
+- Keep offline fixtures representative and safe.
+- Separate governance checkout work from live MailPlus integration work.
+- Improve public-readiness docs and validation gates.
+
+## Non-Goals
+
+- Do not depend on live private mail for routine tests.
+- Do not expose mailbox content in docs, logs, or generated artifacts.
+- Do not turn local intelligence into cloud mailbox surveillance.