fix(grok): recheck native session ownership before rebind

[edwin-zvs]

Summary

• revalidate a newly discovered Grok native session against fresh same-cwd sibling ownership before rebinding
• prevent fresh Grok sessions and usage probes from stealing existing watchers and duplicating or omitting token usage
• preserve legitimate unclaimed native reset discovery and update the ownership specs

Verification

• cargo test -p construct-adapter-grok (41 passed)
• cargo build