Generate 6-char hash IDs with progressive 7/8-char fallback on collision (bd-7c87cf24)

- Changed generateHashID to start with 6 chars (3 bytes), expand to 7/8 on collision
- Updated both CreateIssue and CreateIssues (batch) to use progressive length fallback
- Updated tests to accept 9-11 char IDs (bd- + 6-8 hex chars)
- All new issues now generate with shorter, more readable IDs
- Existing 8-char IDs preserved (no migration needed)

Amp-Thread-ID: https://ampcode.com/threads/T-8a6058af-9f42-4bff-be02-8c8bce41eeb5
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/migrate.go

@@ -279,6 +279,7 @@ This command:
 			}
 		}
 
+
 		// Migrate to hash IDs if requested
 		if toHashIDs {
 			if !jsonOutput {