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/config.rs

@@ -9,7 +9,7 @@
 //
 // Example:
 //   {"config": {"user_name": "Alice", "data_dir": "~/.claude/memory"}}
-//   {"group": "identity", "keys": ["identity.md"]}
+//   {"group": "identity", "keys": ["identity"]}
 //   {"group": "orientation", "keys": ["where-am-i.md"], "source": "file"}
 
 use std::path::PathBuf;
@@ -65,18 +65,18 @@ impl Default for Config {
             assistant_name: "Assistant".to_string(),
             data_dir: home.join(".claude/memory"),
             projects_dir: home.join(".claude/projects"),
-            core_nodes: vec!["identity.md".to_string(), "core-practices.md".to_string()],
+            core_nodes: vec!["identity".to_string(), "core-practices".to_string()],
             journal_days: 7,
             journal_max: 20,
             context_groups: vec![
                 ContextGroup {
                     label: "identity".into(),
-                    keys: vec!["identity.md".into()],
+                    keys: vec!["identity".into()],
                     source: ContextSource::Store,
                 },
                 ContextGroup {
                     label: "core-practices".into(),
-                    keys: vec!["core-practices.md".into()],
+                    keys: vec!["core-practices".into()],
                     source: ContextSource::Store,
                 },
             ],