Make hook dry-runs truthful for install and uninstall

[Widthdom]

Summary

• share install and uninstall planning so successful previews return status: dry_run, filesystem_mutation: false, explicit state/action fields, and hash/provenance-bearing file changes
• add uninstall previews for managed-block removal, chained-hook restoration, forced removal/restoration, blocked conflicts, and idempotent no-op states
• preserve surrounding hook bytes and existing POSIX modes, including non-UTF-8 content, CR-only line endings, UTF BOM variants, and bounded hashing for oversized hooks

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Debug -f net8.0 --no-restore --filter "FullyQualifiedName~HookCommandRunnerTests|FullyQualifiedName~CliFlagSchemaTests" — 48 passed
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Debug -f net9.0 --no-restore --filter "FullyQualifiedName~HookCommandRunnerTests|FullyQualifiedName~CliFlagSchemaTests" — 47 passed, 1 skipped
• full Release net9.0 suite — 10,364 passed, 420 skipped
• full Release net8.0 suite — 10,836 passed, 7 skipped, with two unrelated failures tracked by Stabilize net8 subproject watcher reconciliation for ancestor .gitignore changes #4955 and Stabilize GitHelper timeout regression test under full-suite load #4982; the Stabilize GitHelper timeout regression test under full-suite load #4982 test passed on isolated rerun
• dotnet build CodeIndex.sln -c Release --no-restore — 0 warnings, 0 errors
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• dotnet run --project tools/CodeIndex.Changelog -- check — 53 fragments valid
• Codex adversarial review — two rounds completed; all five P2 findings addressed

Documentation and changelog

• updated the English and Japanese hook sections in USER_GUIDE.md and TESTING_GUIDE.md
• added changelog.d/unreleased/4883.fixed.md

Follow-ups

• Stabilize net8 subproject watcher reconciliation for ancestor .gitignore changes #4955
• Stabilize GitHelper timeout regression test under full-suite load #4982

Fixes #4883