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>

cmd/bd/main.go

@@ -410,9 +410,6 @@ var rootCmd = &cobra.Command{
 		// Warn if multiple databases detected in directory hierarchy
 		warnMultipleDatabases(dbPath)
 
-		// Check for version mismatch (warn if binary is older than DB)
-		checkVersionMismatch()
-
 		// Auto-import if JSONL is newer than DB (e.g., after git pull)
 		// Skip for import command itself to avoid recursion
 		// Skip for delete command to prevent resurrection of deleted issues (bd-8kde)