Version bump to 0.20.1: Hash-based IDs

- Bump version across all components (CLI, plugin, MCP server)
- Update CHANGELOG.md with comprehensive hash ID migration notes
- Replace critical multi-clone warning with hash ID announcement
- Add Hash-Based Issue IDs section to README with:
  - ID format explanation (4/5/6 char progressive scaling)
  - Why hash IDs solve collision issues
  - Birthday paradox collision probability math
  - Migration instructions
- Update all examples to use hash IDs (bd-a1b2) instead of sequential (bd-1)

Breaking changes:
- Sequential ID generation removed (bd-c7af, bd-8e05, bd-4c74)
- issue_counters table removed from schema
- --resolve-collisions flag removed (no longer needed)

Migration: Run 'bd migrate' to upgrade database schema
Amp-Thread-ID: https://ampcode.com/threads/T-0b000145-350a-4dfe-a3f1-67d4d52a6717
Co-authored-by: Amp <amp@ampcode.com>

cmd/bd/version.go

@@ -11,7 +11,7 @@ import (
 
 var (
 	// Version is the current version of bd (overridden by ldflags at build time)
-	Version = "0.20.0"
+	Version = "0.20.1"
 	// Build can be set via ldflags at compile time
 	Build = "dev"
 )