## Why
A malformed plugin manifest still marks a capability root as plugin-owned. Discovery or metadata failures must not be treated as evidence that a root is standalone.
## What changed
- Add `ExecutorPluginProvider::root_ownership` and export `PluginRootOwnership` to distinguish plugin-owned roots from standalone roots before capability parsing.
- Use discovery results when conclusive; otherwise probe manifest files at the selected root and its ancestors using the supplied filesystem sandbox. Nested plugin manifests do not establish ownership of their parent root.
- Share manifest lookup with plugin resolution and pipeline up to 256 metadata probes while preserving ancestor and manifest priority, including error ordering.
## Testing
Add tests for malformed ancestor manifests, discovery failures, ancestor versus nested ownership, concurrent probe ordering, and sandboxed metadata checks without reading manifest contents.
GitOrigin-RevId: 3512452cf1df22f52c2a2b422f96c760d98c24ac
stevenlee-oai
b3b83ace17