Two pre-0.9.0 safety to-dos from the migration spike:
1. Auto-detect-and-warn: graph_has_legacy_ids() samples a loaded graph's node ids
and detects the pre-#1504 (parent-dir/filename) scheme. Read-only consumers that
don't re-extract — `graphify query` and the MCP serve loader — now print a
one-line nudge to rebuild with `extract --force` when they load an old graph.
Fires only on legacy graphs (verified: canonical graphs stay silent).
2. Re-key source_file contract: pinned with tests that a relative source_file is
migrated to the full-path id while an absolute/unrelativizable one is skipped
(its on-disk path must never leak into a persisted id). The is_absolute() guard
already enforced this; the tests make the contract explicit.
Full suite 2507 passed. Lands on v9.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
