Add create-subset command for database subsetting (#162)

## Summary

- Adds a `create-subset` CLI command that copies the N most recent
messages (and all referenced data) from the archive into a new,
standalone msgvault database
- Copies all dependent rows (conversations, participants, labels,
recipients, bodies, attachments) in FK-safe dependency order with
foreign key verification
- Updates denormalized conversation counts and populates FTS index in
the destination database
- Includes tests covering basic subsetting, row limits, FTS population,
conversation count consistency, pre-existing directories, duplicate
destination detection, and SQL injection/control character path
validation

Supersedes #101.

## Test plan

- [x] Unit tests pass (`go test ./internal/store/ -run TestCopySubset`)
- [ ] Manual test: `./msgvault create-subset -o /tmp/subset --rows 100`
against a real archive
- [ ] Verify subset DB works: `MSGVAULT_HOME=/tmp/subset msgvault tui`

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

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: wesm

cmd/msgvault/cmd/create_subset.go

@@ -0,0 +1,100 @@
+package cmd
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"time"
+
+	"github.com/spf13/cobra"
+	"github.com/wesm/msgvault/internal/store"
+)
+
+var createSubsetCmd = &cobra.Command{
+	Use:   "create-subset",
+	Short: "Create a smaller database from the archive",
+	Long: `Create a new msgvault database containing a subset of the
+most recent messages. Useful for testing, demos, or sharing.
+
+The destination directory will contain a complete msgvault.db with
+all referenced data (conversations, participants, labels, etc.)
+and can be used directly:
+
+  MSGVAULT_HOME=/path/to/subset msgvault tui`,
+	RunE: runCreateSubset,
+}
+
+var (
+	subsetOutput string
+	subsetRows   int
+)
+
+func init() {
+	createSubsetCmd.Flags().StringVarP(
+		&subsetOutput, "output", "o", "",
+		"destination directory (msgvault.db created inside)",
+	)
+	createSubsetCmd.Flags().IntVar(
+		&subsetRows, "rows", 0,
+		"number of most recent messages to copy",
+	)
+	_ = createSubsetCmd.MarkFlagRequired("output")
+	_ = createSubsetCmd.MarkFlagRequired("rows")
+	rootCmd.AddCommand(createSubsetCmd)
+}
+
+func runCreateSubset(cmd *cobra.Command, _ []string) error {
+	if err := MustBeLocal("create-subset"); err != nil {
+		return err
+	}
+
+	if subsetRows <= 0 {
+		return fmt.Errorf("--rows must be a positive integer")
+	}
+
+	srcDBPath := cfg.DatabaseDSN()
+	if _, err := os.Stat(srcDBPath); os.IsNotExist(err) {
+		return fmt.Errorf(
+			"source database not found: %s\n"+
+				"Run 'msgvault init-db' and sync first",
+			srcDBPath,
+		)
+	}
+
+	dstDir, err := filepath.Abs(subsetOutput)
+	if err != nil {
+		return fmt.Errorf("resolve output path: %w", err)
+	}
+
+	fmt.Fprintf(os.Stderr,
+		"Copying %d messages from %s...\n", subsetRows, srcDBPath,
+	)
+
+	result, err := store.CopySubset(srcDBPath, dstDir, subsetRows)
+	if err != nil {
+		return fmt.Errorf("create subset: %w", err)
+	}
+
+	fmt.Fprintf(os.Stderr,
+		"Created subset (%s)\n", result.Elapsed.Round(time.Millisecond),
+	)
+	fmt.Printf("Sources:       %d\n", result.Sources)
+	fmt.Printf("Messages:      %d\n", result.Messages)
+	fmt.Printf("Conversations: %d\n", result.Conversations)
+	fmt.Printf("Participants:  %d\n", result.Participants)
+	fmt.Printf("Labels:        %d\n", result.Labels)
+	fmt.Printf("Database size: %s\n", formatSize(result.DBSize))
+
+	if int64(subsetRows) > result.Messages {
+		fmt.Fprintf(os.Stderr,
+			"Note: requested %d messages but source only had %d\n",
+			subsetRows, result.Messages,
+		)
+	}
+
+	fmt.Fprintf(os.Stderr,
+		"\nTo use: MSGVAULT_HOME=%s msgvault tui\n", dstDir,
+	)
+
+	return nil
+}