store: strip .md suffix from all keys

Keys were a vestige of the file-based era. resolve_key() added .md
to lookups while upsert() used bare keys, creating phantom duplicate
nodes (the instructions bug: writes went to "instructions", reads
found "instructions.md").

- Remove .md normalization from resolve_key, strip instead
- Update all hardcoded key patterns (journal.md# → journal#, etc)
- Add strip_md_keys() migration to fsck: renames nodes and relations
- Add broken link detection to health report
- Delete redirect table (no longer needed)
- Update config defaults and config.jsonl

Migration: run `poc-memory fsck` to rename existing keys.

Co-Authored-By: ProofOfConcept <poc@bcachefs.org>

src/query.rs

@@ -17,7 +17,7 @@
 // Examples:
 //   degree > 15 | sort degree | limit 10
 //   category = core | select degree,weight
-//   neighbors('identity.md') WHERE strength > 0.5 | sort strength
+//   neighbors('identity') WHERE strength > 0.5 | sort strength
 //   key ~ 'journal.*' AND degree > 10 | count
 //   * | sort weight asc | limit 20