Update OpenSpec project documentation

[google-labs-jules]

Populated documentation/architecture/openspec/project.md with detailed information about the frigid project, including its purpose, tech stack (Python, Hatch, Ruff, etc.), and conventions.

Key additions:

• Filled in Purpose, Tech Stack, Domain Context, and External Dependencies.
• detailed Project Conventions for Code Style, Architecture Patterns, Testing Strategy, and Git Workflow.
• Added references to documentation/architecture/filesystem.rst, summary.rst, and testplans/index.rst as requested.

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PR created automatically by Jules for task 15875461040734152855 started by @emcd

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[emcd]

Looks good! Left one nitpick.