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/store/ops.rs

@@ -217,14 +217,14 @@ impl Store {
         let cfg = crate::config::get();
         let core_files: Vec<&str> = cfg.core_nodes.iter().map(|s| s.as_str()).collect();
         let tech_files = [
-            "language-theory.md", "zoom-navigation.md",
-            "rust-conversion.md", "poc-architecture.md",
+            "language-theory", "zoom-navigation",
+            "rust-conversion", "poc-architecture",
         ];
         let tech_prefixes = ["design-"];
         let obs_files = [
-            "reflections.md", "reflections-zoom.md", "differentiation.md",
-            "cognitive-modes.md", "paper-notes.md", "inner-life.md",
-            "conversation.md", "interests.md", "stuck-toolkit.md",
+            "reflections", "reflections-zoom", "differentiation",
+            "cognitive-modes", "paper-notes", "inner-life",
+            "conversation", "interests", "stuck-toolkit",
         ];
         let obs_prefixes = ["skill-", "worked-example-"];