Add Fermi sanity check validation gate (#88)

[neoneye]

Summary

• Adds a rule-based validation gate that audits distilled plan assumptions for common failure modes
• Checks for: missing quantification (no numbers/bounds), implausibly wide range spans (>100x ratio), and low-confidence language without evidence references
• Runs as a pipeline task (FermiSanityCheckTask) after DistillAssumptionsTask — no LLM calls, fast execution
• Outputs both raw JSON and markdown report (003-12-fermi_sanity_check_raw.json, 003-13-fermi_sanity_check.md)

Implements proposal #88.

Test plan

• Syntax check passes for all modified files
• Standalone module runs correctly with sample assumptions
• Pipeline integration test in Docker

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