Add bd config command for external integration configuration

- Add GetAllConfig/DeleteConfig methods to storage interface
- Implement config set/get/list/unset subcommands with JSON support
- Add comprehensive tests for config operations
- Create CONFIG.md with full documentation and examples
- Update README.md with config section
- Support namespace conventions (jira.*, linear.*, github.*, custom.*)

Closes bd-60

Amp-Thread-ID: https://ampcode.com/threads/T-33db7481-de7c-475e-b562-6afb7fb4bc7a
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/config.go

@@ -0,0 +1,175 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"sort"
+
+	"github.com/spf13/cobra"
+)
+
+var configCmd = &cobra.Command{
+	Use:   "config",
+	Short: "Manage configuration settings",
+	Long: `Manage configuration settings for external integrations and preferences.
+
+Configuration is stored per-project in .beads/*.db and is version-control-friendly.
+
+Common namespaces:
+  - jira.*     Jira integration settings
+  - linear.*   Linear integration settings
+  - github.*   GitHub integration settings
+  - custom.*   Custom integration settings
+
+Examples:
+  bd config set jira.url "https://company.atlassian.net"
+  bd config set jira.project "PROJ"
+  bd config get jira.url
+  bd config list
+  bd config unset jira.url`,
+}
+
+var configSetCmd = &cobra.Command{
+	Use:   "set <key> <value>",
+	Short: "Set a configuration value",
+	Args:  cobra.ExactArgs(2),
+	Run: func(cmd *cobra.Command, args []string) {
+		// Config operations work in direct mode only
+		if err := ensureDirectMode("config set requires direct database access"); err != nil {
+			fmt.Fprintf(os.Stderr, "Error: %v\n", err)
+			os.Exit(1)
+		}
+
+		key := args[0]
+		value := args[1]
+
+		ctx := context.Background()
+		if err := store.SetConfig(ctx, key, value); err != nil {
+			fmt.Fprintf(os.Stderr, "Error setting config: %v\n", err)
+			os.Exit(1)
+		}
+
+		if jsonOutput {
+			outputJSON(map[string]string{
+				"key":   key,
+				"value": value,
+			})
+		} else {
+			fmt.Printf("Set %s = %s\n", key, value)
+		}
+	},
+}
+
+var configGetCmd = &cobra.Command{
+	Use:   "get <key>",
+	Short: "Get a configuration value",
+	Args:  cobra.ExactArgs(1),
+	Run: func(cmd *cobra.Command, args []string) {
+		// Config operations work in direct mode only
+		if err := ensureDirectMode("config get requires direct database access"); err != nil {
+			fmt.Fprintf(os.Stderr, "Error: %v\n", err)
+			os.Exit(1)
+		}
+
+		key := args[0]
+
+		ctx := context.Background()
+		value, err := store.GetConfig(ctx, key)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "Error getting config: %v\n", err)
+			os.Exit(1)
+		}
+
+		if jsonOutput {
+			outputJSON(map[string]string{
+				"key":   key,
+				"value": value,
+			})
+		} else {
+			if value == "" {
+				fmt.Printf("%s (not set)\n", key)
+			} else {
+				fmt.Printf("%s\n", value)
+			}
+		}
+	},
+}
+
+var configListCmd = &cobra.Command{
+	Use:   "list",
+	Short: "List all configuration",
+	Run: func(cmd *cobra.Command, args []string) {
+		// Config operations work in direct mode only
+		if err := ensureDirectMode("config list requires direct database access"); err != nil {
+			fmt.Fprintf(os.Stderr, "Error: %v\n", err)
+			os.Exit(1)
+		}
+
+		ctx := context.Background()
+		config, err := store.GetAllConfig(ctx)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "Error listing config: %v\n", err)
+			os.Exit(1)
+		}
+
+		if jsonOutput {
+			outputJSON(config)
+			return
+		}
+
+		if len(config) == 0 {
+			fmt.Println("No configuration set")
+			return
+		}
+
+		// Sort keys for consistent output
+		keys := make([]string, 0, len(config))
+		for k := range config {
+			keys = append(keys, k)
+		}
+		sort.Strings(keys)
+
+		fmt.Println("\nConfiguration:")
+		for _, k := range keys {
+			fmt.Printf("  %s = %s\n", k, config[k])
+		}
+	},
+}
+
+var configUnsetCmd = &cobra.Command{
+	Use:   "unset <key>",
+	Short: "Delete a configuration value",
+	Args:  cobra.ExactArgs(1),
+	Run: func(cmd *cobra.Command, args []string) {
+		// Config operations work in direct mode only
+		if err := ensureDirectMode("config unset requires direct database access"); err != nil {
+			fmt.Fprintf(os.Stderr, "Error: %v\n", err)
+			os.Exit(1)
+		}
+
+		key := args[0]
+
+		ctx := context.Background()
+		if err := store.DeleteConfig(ctx, key); err != nil {
+			fmt.Fprintf(os.Stderr, "Error deleting config: %v\n", err)
+			os.Exit(1)
+		}
+
+		if jsonOutput {
+			outputJSON(map[string]string{
+				"key": key,
+			})
+		} else {
+			fmt.Printf("Unset %s\n", key)
+		}
+	},
+}
+
+func init() {
+	configCmd.AddCommand(configSetCmd)
+	configCmd.AddCommand(configGetCmd)
+	configCmd.AddCommand(configListCmd)
+	configCmd.AddCommand(configUnsetCmd)
+	rootCmd.AddCommand(configCmd)
+}