refactor(team-planex): redesign skill with inverted control and beat model

- Delete executor agent (main flow IS the executor now)
- Rewrite SKILL.md: delegated planning + inline execution
- Input accepts issues.jsonl / roadmap session from roadmap-with-file
- Single reusable planner agent via send_input (Pattern 2.3)
- Interleaved plan-execute loop with eager delegation
- Follow codex v3 conventions (decision tables, placeholders)
- Remove complexity assessment and dynamic splitting

Author: catlog22