docs: correct config discovery boundary - #45681

Why

The shipped OpenCode skill and its linked configuration guide say project config discovery stops at the project root. The loader intentionally walks every ancestor through the filesystem root, and an existing Core test preserves configuration found above the detected project boundary.

What Changes

• Describes ordinary project discovery as walking the current Location and all filesystem ancestors, including directories above the project or repository root.
• Retains the existing farthest-to-nearest ordering for direct files followed by discovered .opencode files.
• Clarifies the relative precedence of global filesystem configuration without presenting it as exhaustive across every explicit or virtual source.

Scope

This corrects CONTRIBUTOR-01 in the built-in skill and companion public guide only. It does not change Config discovery, watcher behavior, opt-outs, source precedence, or the separate instruction-discovery boundary.

Verification

cd packages/core
bun typecheck

cd ../www
bun typecheck
bun run check:generated
bun run build

cd ../..
bunx prettier --check packages/core/src/plugin/skill/opencode.md packages/www/src/docs/content/config.mdx
bun run typecheck
git diff --check

All checks passed. The website build completed link validation, and the repository-wide typecheck completed all 33 tasks.