Restructures ce:plan around a decisions-first philosophy:
- Replace issue-template output with durable implementation plans
- Add blocker classification gate for upstream requirements (R11-R13)
- Replace MINIMAL/MORE/A LOT with Lightweight/Standard/Deep
- Add planning bootstrap fallback with ce:brainstorm recommendation
- Remove all implementation code, shell commands, and executor litter
- Make SpecFlow conditional for Standard/Deep plans
- Keep research agents, brainstorm-origin integration, and handoff options
- Restore origin doc completeness checks, user signal gathering,
research decision examples, filename examples, stakeholder awareness,
and mermaid diagram nudges from the old skill
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