[codex] Document resource budget diagnostics

[zergzorg]

Summary

Part of #487.

This PR adds a resource-budget diagnostics guide for maintainers and contributors. It is intentionally docs-only so it does not interfere with the open code PRs in the #487 stack.

Changes:

• add docs/resource-budget.md with repeatable commands for idle CPU/memory/thread baselines, long idle watch, child process watch, local HTTP API stress, and provider probe timing;
• document expected healthy signals without treating any single CPU/RAM number as a hard limit;
• add a PR review checklist for future background refresh/helper-command changes;
• link the guide from the README.

This covers the #487 acceptance item for docs/maintainer notes describing expected idle behavior and measurement steps. It is independent from #498/#499/#500/#501/#502/#503 and can merge anytime.

Verification

• git diff --check

Docs-only change; no runtime code or generated assets changed.

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Summary by cubic

Adds a resource-budget diagnostics guide with repeatable commands for checking idle CPU, memory, threads, child processes, local HTTP API stress, and provider timing. Links it from the README and adds a PR review checklist; meets #487’s docs acceptance item.

^{Written for commit f7d61ce. Summary will update on new commits. Review in cubic}

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}