* fix(harness): resolve runtime paths from project root
* docs(config): update
* fix(config): address runtime path review feedback
* test(config): fix skills path e2e root
* test(config): cover legacy config fallback when project root lacks config files
Verifies that when DEER_FLOW_PROJECT_ROOT is unset and cwd has no
config.yaml/extensions_config.json, AppConfig and ExtensionsConfig fall back
to the legacy backend/repo-root candidates — the backward-compat path
requested in PR #2642 review.
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Co-authored-by: Willem Jiang <willem.jiang@gmail.com>