Rename upstream import policy docs

[cbusillo]

Summary

• rename stale docs/local-overlay.md to docs/upstream-import-policy.md and update repo metadata/routing references
• clarify that Every Code owns main and imports from upstream sources rather than living as a local overlay branch
• rename local upstream import helper/manifest away from overlay terminology while preserving just local-upstream-import

Validation

• rg -n "docs/local-overlay\.md|local-overlay\.md|upstreamImportPolicy" AGENTS.md README.md docs .github scripts justfile
• rg -n "update-overlay-from-upstream|overlay-picks|local-overlay-update|local-overlay|docs/local-overlay|local overlay|overlay branch|outside the overlay" AGENTS.md docs scripts justfile .github/github.json
• git diff --check
• ./build-fast.sh

Refs #179
Refs #184