fix: ignore shared AGENTS root in rules check

[ProfTrader]

Summary

• Skip project-root AGENTS.md during rules --check for modular rule targets when non-root rule outputs are present.
• Keep normal generation unchanged, and keep Codex memory files checked.
• Add unit and e2e coverage for the shared-root AGENTS.md case.

Fixes #1894

Verification

• pnpm exec vitest run src/features/rules/rules-processor.test.ts -t "ignore shared root AGENTS.md" --silent=true
• pnpm exec oxfmt --check src/features/rules/rules-processor.test.ts src/features/rules/rules-processor.ts src/lib/generate.ts src/e2e/e2e-rules.spec.ts
• pnpm exec oxlint src/features/rules/rules-processor.test.ts src/features/rules/rules-processor.ts src/lib/generate.ts src/e2e/e2e-rules.spec.ts --max-warnings 0
• pnpm run typecheck

Notes

• Full src/e2e/e2e-rules.spec.ts is blocked locally on Windows because the e2e helper spawns node_modules/.bin/tsx without the .cmd shim, so every e2e case fails before reaching this change.
• The full rules-processor.test.ts file currently has an unrelated failure on this branch: should generate copilot global non-root files via round-trip.