Add restore dry-run preview

[olxgdm]

Summary

Closes #77.

This PR adds a non-mutating restore preview mode for both single-file and batch restore flows:

• cfgsync restore <file> --dry-run
• cfgsync restore --all --dry-run
• prefix-remapped dry-runs with --from-prefix / --to-prefix

Dry-run validates the same restore inputs that apply mode depends on, including tracked registry entries, stored backup paths, ordinary-file backups, prefix remapping, and destination state. It then prints one impact line per planned restore without copying files, creating destination parent directories, or mutating registry/storage/app config/original files.

Behavior

Dry-run emits machine-readable preview lines:

would-create <destination>
would-overwrite <destination>
unchanged <destination>

The command class now uses an explicit RestoreMode { Apply, DryRun }, keeping real file copying in StorageManager::RestoreEntry and keeping dry-run planning in RestoreCommand.

Batch dry-run preserves existing restore batch semantics: recoverable per-file failures are reported, later registry entries continue to be processed, and the command exits non-zero with the existing Restore completed with N failure(s). summary.

Implementation Notes

• Adds --dry-run to the restore CLI.
• Reuses existing restore prefix-remap behavior.
• Validates stored backups through StorageManager::ResolveStoredPath and ordinary-file checks.
• Compares stored and destination files byte-for-byte to distinguish unchanged from would-overwrite.
• Treats existing non-ordinary destinations as invalid destination state.
• Updates README restore and fresh-system examples to show preview-before-apply workflows.

Tests

Added command-level and CLI-level coverage for:

• single-file dry-run preserving changed destination contents
• batch dry-run preserving multiple destinations
• would-create, would-overwrite, and unchanged output
• prefix-remapped dry-run without destination directory creation
• missing stored backup failures
• batch dry-run continuation after an outside-prefix failure
• restore --help documenting --dry-run

Verification

Ran locally:

cmake --build build
ctest --test-dir build -R "RestoreCommand|RestoreCommandCli" --output-on-failure
ctest --test-dir build --output-on-failure
ctest --test-dir build --output-on-failure --quiet

All tests passed, including the full CTest suite: 260/260.

[sonarqubecloud]

Quality Gate failed

Failed conditions
71.9% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud