fix(parser): warn when a task has no **Files:** section at all

[bacsystem]

Summary

Twin of the 0.6.22 **Interfaces:** fix, same root cause, other section: parseFiles still used extractSection, which flattens "section missing" and "section present but empty" to ''. A header typo (**File:**, **Archivos:**) silently removed the task from file-based serialization — two tasks sharing a file could run in parallel and hit an avoidable merge conflict, with a misleading symptom (looks like a Consumes/Produces problem, not a Files one).

Type of change

• fix — bug fix

Main changes

• parseFiles now uses extractOptionalSection (same helper parseInterfaces already uses) and warns when the **Files:** section is missing entirely.
• extractSection is now unused (both parseFiles and parseInterfaces use the optional variant) and has been removed.
• skills/plan/SKILL.md: extended the 0.6.22 rule to cover both **Files:** and **Interfaces:** headers.
• Verified plan-parser.js is not inlined into the generated workflow (npm run build + git diff --exit-code workflows/parallel-plan-executor.js — no change).

Version

• Bump: patch (diagnostics-only, no API or graph change)
• New version: v0.6.23

Checklist

• Code reviewed (bugs, conventions, simplifications)
• CHANGELOG.md updated
• Version updated in the relevant file(s) (package.json, .claude-plugin/plugin.json, .cursor-plugin/plugin.json, gemini-extension.json)
• README updated (if applicable) — no user-facing behavior/API change
• Tests pass (163/163, npm test), TDD (RED confirmed before GREEN)