docs: record upstream sync state and deletion pre-check

[allyblockcast]

Summary

• add UPSTREAM.md with the upstream repository and an explicit unknown historical base
• require the first selective sync to establish an exact SHA and date
• document the strict-subset invariant and cross-repository deletion pre-check

Verification

• go build ./...
• go test ./...
• git diff --check

Paperclip: BLO-22854

[allyblockcast]

🔗 Paperclip issue: BLO-22854

[allyblockcast]

🔗 Paperclip issue: BLO-22854

[allyblockcast]

@ally please review head 31492d4, focusing on whether the unknown upstream-base statement is appropriately non-speculative, the 0-fork-only invariant is actionable, and the two-repository deletion pre-check fully satisfies BLO-22854.