Normalize claim remotes and scratch failures

[Deicyde]

Summary

• recognize SCP-style Git remotes with or without an explicit user@ prefix
• keep rooted, device/UNC, and Windows drive-relative repository paths local instead of treating their drive letter as an SSH host
• translate every scratch readiness/creation failure into ClaimTransportError, including an inaccessible parent encountered before mkdir
• retry concurrent first-time bare-repository initialization until one complete shared scratch is verifiably ready

Before this change, github-work:org/repo.git was resolved as a local path, while Windows C:claims.git was misclassified as an SCP remote and used relative to the scratch directory. An inaccessible scratch path could also escape as a raw PermissionError, and concurrent first use frequently collided on Git's config.lock.

Validation

At exact head 6aede87a:

• full claims suite: 72 passed
• five repeated 12-thread first-use stress runs: pass
• Python 3.10 remote/scratch/concurrency regressions: 14 passed
• Ruff, diff check, and make check-example: pass
• exact-head GitHub Python 3.10/3.13, Windows, real Lean, and CLA: all pass

#137 has landed the former README baseline fix.

Fixes F33 and F34 from #92's adversarial review.

[Deicyde]

Exact-head review is complete at 6aede87a. Windows drive-relative and device paths remain local, inaccessible readiness probes are wrapped, and concurrent first use now converges on one verified bare scratch. Python 3.10/3.13, Windows, both real-Lean runs, CLA, the 72-test claims suite, repeated concurrency stress, lint, and the executable example all pass. No known blocker remains.