fix(pm): make remote reachability a measured precondition of the governed-merge sweep (#13421)

* fix(pm): make remote reachability a measured precondition of the governed-merge sweep

A governed repo whose checkout is present and healthy but whose remote is
unreachable was audited from the local snapshot and reported `0 mainline
commit(s) in window` — a false zero that could never advance, because the
remedy the row prescribed was the command that failed. One `git ls-remote`
per repo now answers reachability and mirror freshness together, feeding the
existing UNAUDITED refusal register rather than a second mechanism.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Pk26oZ12t5N1hwGW1m1MgC

* test(pm): pin the reachability leg end to end, including the sweep's own call site

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Pk26oZ12t5N1hwGW1m1MgC

* test(pm): report reachability-pin casualties instead of throwing on the first

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Pk26oZ12t5N1hwGW1m1MgC

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Co-authored-by: Claude <noreply@anthropic.com>

Author: os-project-manager