fix(formula): address code review findings

Fixes from self-review of formula parser and bd cook:

1. Atomicity: Add cleanup on failure in cookFormula - if labels/deps
   transaction fails, delete the already-created issues

2. Validation: Add recursive depends_on validation for child steps
   and validate priority ranges for children

3. Documentation: Mark unimplemented fields (Expand, ExpandVars,
   Condition, Gate) with TODO(future) comments

4. Thread safety: Add note that Parser is NOT thread-safe

5. Error messages: Track first definition location for duplicate ID
   errors (e.g., "duplicate id at steps[1], first defined at steps[0]")

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

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

internal/formula/parser.go

@@ -14,6 +14,10 @@ import (
 const FormulaExt = ".formula.yaml"
 
 // Parser handles loading and resolving formulas.
+//
+// NOTE: Parser is NOT thread-safe. Create a new Parser per goroutine or
+// synchronize access externally. The cache and resolving maps have no
+// internal synchronization.
 type Parser struct {
 	// searchPaths are directories to search for formulas (in order).
 	searchPaths []string