Add filesystem loading for frameworks and clarify starter definitions

[davidahmann]

Summary

• add runtime filesystem loading for framework YAML via framework.Load/proof.LoadFramework(path) while preserving built-in embedded framework IDs
• update proof frameworks show help text to support id-or-path and add a human summary line to proof frameworks list
• align framework positioning/docs as built-in starter definitions and remove overclaims in README
• add tests for filesystem framework loading in core/API/CLI paths

Validation

• go test ./...
• go vet ./...

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve embedded lookup before filesystem probing

Checking os.Stat(idOrFile) before embedded resolution makes built-in framework IDs/file names depend on the caller’s current working directory: any local file named like a built-in (for example eu-ai-act.yaml) will be loaded instead of the embedded starter definition. This is a regression introduced here because List() calls Load() with embedded file names, so proof frameworks list can silently show user-local data or fail if that local file is invalid, breaking deterministic built-in behavior.

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