Adds the framework for evolving the prefs schema without silently breaking
projects pinned to older versions. Each PREFERENCES.md declares `version: N`;
sf declares CURRENT_PREFERENCES_SCHEMA_VERSION in code. On load:
- prefs.version === current → no-op
- prefs.version < current → run registered migrations in chain (forward only,
pure functions). Missing migration in the chain throws — bumping the
schema version requires a matching Migration entry, by construction.
- prefs.version > current → warn "prefs from a newer sf, fields may be
ignored", preserve the value so a later upgrade reads correctly.
- prefs.version undefined → assume v1 (legacy file pre-versioning) and
warn so the user adds an explicit pin.
Migration registry is empty for now (current schema version stays at 1) —
the framework is in place so the first real schema bump is a one-line
addition, not a refactor. Drift detection (`checkPreferencesDrift`) is also
the natural surface for future deprecated-key / missing-required-field
checks when CLAUDE.md / template comparisons are added.
Wired into validatePreferences() so every load path gets the new behavior
automatically — no caller changes needed.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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