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/llm.rs

@@ -137,11 +137,11 @@ pub(crate) fn parse_json_response(response: &str) -> Result<serde_json::Value, S
 pub(crate) fn semantic_keys(store: &Store) -> Vec<String> {
     let mut keys: Vec<String> = store.nodes.keys()
         .filter(|k| {
-            !k.starts_with("journal.md#")
-            && *k != "journal.md"
-            && *k != "MEMORY.md"
-            && *k != "where-am-i.md"
-            && *k != "work-queue.md"
+            !k.starts_with("journal#")
+            && *k != "journal"
+            && *k != "MEMORY"
+            && *k != "where-am-i"
+            && *k != "work-queue"
             && *k != "work-state"
         })
         .cloned()