fix: clarify Knowledge Legacy is local-record, not per-domain backup exports

[atomantic]

Summary

• Issue [plan-feature-brain-export] Export Brain captures to Markdown + JSON #6069 (Brain Markdown/JSON backup export plan) is closed as not planned: PortOS is a locally hosted app that is itself the user's backup of record (local data + automatic snapshots), so per-domain backup-style exports are out of scope.
• Root-cause doc fix: PRD.md Goal 9 said 'durable, exportable record' (misread as a backup-export mandate). Now reads 'durable, local record' with the backup philosophy stated inline.
• New NR-10 negative requirement forbids per-domain backup-style export endpoints; permitted exports (Sharing buckets, Legacy Bundle, format-specific creative deliverables) are named explicitly.
• Matching one-sentence clarification in GOALS.md Goal 9 and an Out of Scope bullet in PRD.md.

Test plan

• Docs-only change: verified git diff touches only GOALS.md and PRD.md wording.
• Local reviewer (claude, sonnet-4-6): 1 round; 2 consistency findings, both applied verbatim.
• Closed [plan-feature-brain-export] Export Brain captures to Markdown + JSON #6069 with a not-planned comment pointing at this correction.