feat(fan-flames): add prior wave context and formatter step to agent prompts

[muloka]

Summary

• Add prior wave context section to Phase 2 dispatch template — Wave 2+ agents get a summary of what prior waves built (files, functions, APIs) so they build on top of existing work instead of conflicting
• Add formatter/linter step to agent self-review checklist (e.g., cargo fmt, prettier, ruff)

Why

Real-world UAT showed Wave 2 agent didn't see Wave 1's decompose_spacing function despite correct sequencing (op log confirmed 2-minute gap between Wave 1 squash and Wave 2 dispatch). The workspace had the code, but the agent wasn't told about it. Prior wave context in the prompt fixes this.

Formatter step addresses agents producing non-standard formatting that required manual cargo fmt after merge.

Test plan

• Run fan-flames on a multi-wave plan, verify Wave 2 agent prompt includes Wave 1 summary
• Verify agents run formatter before reporting

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