Add issue-prefix as a config.yaml option

Allow setting issue prefix via config.yaml that works independently
of --no-db mode. This provides a consistent way to set the prefix
across the entire repository.

Precedence order:
1. --prefix flag (highest)
2. issue-prefix in config.yaml
3. .beads/nodb_prefix.txt (no-db mode only)
4. Auto-detect from directory name (lowest)

Changes:
- Add issue-prefix to config defaults in internal/config/config.go
- Update cmd/bd/init.go to read from config before auto-detecting
- Update cmd/bd/nodb.go detectPrefix to check config.yaml
- Update .beads/config.yaml with documentation and example

Usage:
  # .beads/config.yaml
  issue-prefix: "myproject"

  # Or via environment variable
  BD_ISSUE_PREFIX=myproject bd init

This makes the prefix setting repository-scoped and automatically
respected by bd init in both normal and no-db modes.

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

internal/config/config.go

@@ -74,6 +74,7 @@ func Initialize() error {
 	v.SetDefault("no-db", false)
 	v.SetDefault("db", "")
 	v.SetDefault("actor", "")
+	v.SetDefault("issue-prefix", "")
 	
 	// Additional environment variables (not prefixed with BD_)
 	// These are bound explicitly for backward compatibility