fix(sync): remote wins on first sync for create_if_missing files

[stefanzvonar]

Why

When a dotfile with create_if_missing = true is added to the config and synced to a new machine, the remote content may not be applied — leaving the file empty or with a default value instead of the synced content.

This happens when an application creates the file before tether runs (e.g. Claude Code writes {} to ~/.claude/settings.json on startup). Tether's conflict detection treats that locally-created default as a "local edit" and refuses to overwrite it with the remote version.

As-Is

1. Machine A adds .claude/settings.json (with real content) to tether config and syncs
2. Machine B runs tether sync — config is pulled, file doesn't exist locally
3. Before decrypt_from_repo writes the remote content, the app creates a default file (e.g. {})
4. detect_conflict() sees: local exists ({}), no last_synced_hash, local ≠ remote → true conflict
5. In daemon mode: conflict is skipped. In interactive mode: user is prompted for a file they never edited
6. Even without a conflict, the "no conflict" path checks local_unchanged = (local_hash == last_synced_hash) → Some(hash) == None → false → remote not written

Both code paths fail to apply remote content when there's no sync history for the file.

To-Be

When create_if_missing = true AND there's no sync history for the file on this machine (last_synced_hash is None), treat it as a first-time sync where remote wins:

1. Skip conflict detection entirely — there's no local state worth preserving
2. Write remote content unless local already matches remote (idempotent)
3. Back up any existing local file before overwriting (safety net)
4. Create parent directories if needed

After the first sync completes, subsequent syncs use the normal three-way conflict detection since last_synced_hash will be populated.

Test plan

• cargo clippy --all-targets -- -D warnings passes
• cargo test passes
• Add .claude/settings.json with create_if_missing = true on Machine A, sync
• On Machine B (where Claude Code already created {}), run tether sync → file should have Machine A's content
• On Machine B, edit the file, sync → local edit preserved (normal conflict detection)
• On Machine C (file doesn't exist at all), run tether sync → file created with remote content

[stefanzvonar]

Superseded by #8 which combines this fix with additional fixes for the same issue area (is_true_conflict + effective_dotfiles merge).