feat: Add version mismatch detection for outdated binaries

Prevent user confusion when running outdated bd binaries by detecting
version mismatches between the binary and database.

Features:
- Store bd version in metadata table on init
- Check version on every command (PersistentPreRun)
- Warn if binary is outdated with rebuild instructions
- Auto-upgrade database if binary is newer
- Silent operation when versions match

Fixes confusion from bd-182 (auto-export not working with old binary)
Implements bd-197

Files changed:
- cmd/bd/init.go: Store version on init
- cmd/bd/main.go: checkVersionMismatch() + integration

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

cmd/bd/init.go

@@ -56,6 +56,12 @@ and database file. Optionally specify a custom issue prefix.`,
 			os.Exit(1)
 		}
 
+		// Store the bd version in metadata (for version mismatch detection)
+		if err := store.SetMetadata(ctx, "bd_version", Version); err != nil {
+			fmt.Fprintf(os.Stderr, "Warning: failed to store version metadata: %v\n", err)
+			// Non-fatal - continue anyway
+		}
+
 		if err := store.Close(); err != nil {
 			fmt.Fprintf(os.Stderr, "Warning: failed to close database: %v\n", err)
 		}