Add explicit backup modes for cfgsync backup

[olxgdm]

Summary

• Add explicit cfgsync backup modes for the behavior requested in issue Add explicit backup modes for missing-only, refresh, and force behavior #75.
• Preserve the default refresh-on-change behavior while adding --missing-only and --force opt-in modes.
• Update CLI help, README guidance, unit tests, and CLI tests for the new mode surface.

Why

The project history made backup semantics ambiguous: one workflow wanted only missing stored copies, while the current v0.3.1 behavior refreshes changed backups. This PR makes those choices explicit without changing the registry format, app config format, storage layout, restore flow, snapshots, or history model.

User impact

• cfgsync backup continues to create missing backups, refresh changed backups, and skip matching backups.
• cfgsync backup --missing-only creates only missing stored backups and preserves existing stored copies.
• cfgsync backup --force overwrites every stored backup from the current tracked originals.
• cfgsync backup --missing-only --force fails with a clear CLI validation error.

Implementation notes

• Introduces BackupMode and keeps BackupCommand::Execute() defaulting to refresh-on-change.
• Keeps batch backup behavior unchanged: per-file failures are warned, processing continues, and the command exits non-zero after the batch when failures occurred.
• Keeps --missing-only from inspecting original files when a stored backup already exists, so existing backups can be preserved even if the original is currently missing.

Validation

• cmake --build build
• ctest --test-dir build --output-on-failure -R "BackupCommand|BackupCommandCli" (29/29 passed)
• ctest --test-dir build --output-on-failure (251/251 passed)
• build/cfgsync backup --help

Closes #75

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
90.9% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud