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/cmd/gitinit.go

@@ -109,7 +109,7 @@ const HQGitignore = `# Gas Town HQ .gitignore
 # Explicitly track (override above patterns)
 # =============================================================================
 # Note: .beads/ has its own .gitignore that handles SQLite files
-# and keeps issues.jsonl, metadata.json, config.yaml as source of truth
+# and keeps issues.jsonl, metadata.json, config file as source of truth
 `
 
 func runGitInit(cmd *cobra.Command, args []string) error {