Remove version field from metadata.json

- Removes noisy version mismatch warnings on every bd upgrade
- Version field in metadata.json was redundant with daemon version checking via RPC
- Daemon version mismatches still detected via HealthResponse
- Removes checkVersionMismatch() function and related test file
- Updates .beads/.gitignore to properly ignore merge artifacts

Amp-Thread-ID: https://ampcode.com/threads/T-7ba8aff2-97a0-4d0c-9008-e858bdfadd61
Co-authored-by: Amp <amp@ampcode.com>

.beads/.gitignore

@@ -18,3 +18,7 @@ bd.db
 !*.jsonl
 !metadata.json
 !config.json
+
+# 3-way merge snapshot files (local-only, for deletion tracking)
+beads.base.jsonl
+beads.left.jsonl

.beads/metadata.json

@@ -1,5 +1,4 @@
 {
   "database": "beads.db",
-  "version": "0.23.0",
   "jsonl_export": "beads.jsonl"
 }