🤖 perf: simplify ssh runtime scaling path

Collapse the SSH scaling redesign around a simpler sharded OpenSSH transport,
hashed project layout, and higher-level integration coverage.

Delete the explicit OpenSSH master pool and project sync coordinator, remove
remote branch-metadata persistence and legacy layout inference, and keep a
serialized per-project sync path with current-snapshot reuse validation.

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_Generated with `mux` • Model: `openai:gpt-5.4` • Thinking: `xhigh` • Cost: `$219.50`_

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Author: ammar-agent