feat(completion): optimize ID prefix filtering and add completions to more commands

Improvements to shell completions from PR #935:

1. Add IDPrefix field to IssueFilter for efficient database-level filtering
   - Queries are now filtered at SQL level instead of fetching all issues
   - Updated sqlite, transaction, and memory stores to support IDPrefix

2. Add ValidArgsFunction to additional commands:
   - dep (add, remove, list, tree)
   - comments, comment (add)
   - delete
   - graph
   - label (add, remove, list)
   - duplicate, supersede
   - audit
   - move
   - relate, unrelate
   - refile
   - gate (show, resolve, add-waiter)

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Executed-By: beads/crew/dave
Rig: beads
Role: crew

cmd/bd/duplicate.go

@@ -50,10 +50,12 @@ var (
 func init() {
 	duplicateCmd.Flags().StringVar(&duplicateOf, "of", "", "Canonical issue ID (required)")
 	_ = duplicateCmd.MarkFlagRequired("of") // Only fails if flag missing (caught in tests)
+	duplicateCmd.ValidArgsFunction = issueIDCompletion
 	rootCmd.AddCommand(duplicateCmd)
 
 	supersedeCmd.Flags().StringVar(&supersededWith, "with", "", "Replacement issue ID (required)")
 	_ = supersedeCmd.MarkFlagRequired("with") // Only fails if flag missing (caught in tests)
+	supersedeCmd.ValidArgsFunction = issueIDCompletion
 	rootCmd.AddCommand(supersedeCmd)
 }