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>

.beads/config.yaml

@@ -3,6 +3,11 @@
 # All settings can also be set via environment variables (BD_* prefix)
 # or overridden with command-line flags
 
+# Issue prefix for this repository (used by bd init)
+# If not set, bd init will auto-detect from directory name
+# Example: issue-prefix: "myproject" creates issues like "myproject-1", "myproject-2", etc.
+# issue-prefix: ""
+
 # Use no-db mode: load from JSONL, no SQLite, write back after each command
 # When true, bd will use .beads/issues.jsonl as the source of truth
 # instead of SQLite database