fix: Remove YAML usage, standardize on JSON/TOML

- Delete 5 legacy .formula.yaml files (have .toml replacements)
- Remove unused FileConfigYAML constant
- Add TODO for beads config.yaml → config.json migration (bd-10wg)
- Update docs to use JSON examples instead of yaml code blocks
- Change plugin frontmatter from YAML to TOML in docs
- Add .toml to code file detection in branch_check.go

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

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

internal/constants/constants.go

@@ -70,9 +70,6 @@ const (
 	// FileConfigJSON is the general config file.
 	FileConfigJSON = "config.json"
 
-	// FileConfigYAML is the beads config file.
-	FileConfigYAML = "config.yaml"
-
 	// FileAccountsJSON is the accounts configuration file in mayor/.
 	FileAccountsJSON = "accounts.json"
 )