#1731 LUKE-334 PRIVACY.md: disclose feature plans

Do not merge yet. The wording is waiting on the user's keep / reword / drop decision, which the orchestrator will relay.

This splits the privacy change out of #1730 (the plan store and the update_plan tool for LUKE-334) so the code doesn't wait on the wording.

Named feature plans are new user data on the hosted service. This adds a Feature plans paragraph under "What we collect", placed before "Luke's working memory". It covers:

• what is stored: the plan's name, the repository owner/name, the default branch and starting commit, and one document (Markdown body plus assumptions, each marked confirmed or not)
• that it is stored as written, bound to the account and readable by operators, like the conversation
• that a save replaces the document and keeps no earlier version
• that deleting a plan removes it at once, and deleting the account removes every plan

Documentation only; no code or Mac/UI change.

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