fix(repo): harden repository state persistence

[GaTTGeng]

What changed

• create atomic replacement files with their final Unix mode so credential-bearing repository config is owner-only from first visibility
• preserve existing Unix modes and re-check cancellation before atomic replacement
• align default repository cache paths with Helm on Linux, macOS, and Windows while preserving HELM/XDG/explicit precedence
• persist raw-URL index updates for every configured duplicate-URL identity
• derive portable, deterministic cache filenames that keep sanitized and case-distinct repository identities isolated
• document the path, identity, permission, and duplicate behavior in English and Chinese

Audit note

The issue's candidate branch was not actually missing from master: PR #167's squash tree (a20f5a2) is identical to the candidate tip (d2e1c71). This PR therefore adds only the remaining gaps found against current master, without replaying the stale commits.

Verification

• dotnet test HelmSharp.sln --configuration Release --no-build --no-restore with localhost excluded from the configured proxy (net8/9/10: 534/534 each)
• repository configuration tests (net8/9/10: 49/49 each)
• repository/update/request/dependency focused tests (net8/9/10: 60/60 each)
• dotnet pack HelmSharp.sln --configuration Release --no-build --output artifacts/packages-local (all packages and symbols)
• git diff origin/master...HEAD --check

Fixes #181

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.