Harden sandbox OAuth model preflight and improve audit report readability

[anand-testcompare]

Summary

• Sync OpenCode OAuth auth state (~/.local/share/opencode/auth.json) into each sandbox with strict permissions, so provider sessions can be reused without relying only on API keys.
• Add Anthropic provider preflight (opencode models anthropic) and fail-fast model availability checks, including dotted/hyphenated model-id normalization fallback.
• Make generated audit reports easier to consume by tightening summary guidance, allowing sentence fragments, and requiring an ASCII logic/data-flow walkthrough section.

Testing

• bun run check
• bun run typecheck
• bun test
• bun run analyze -- --out-dir findings-anthropic-oauth-proof3 --model "anthropic/claude-sonnet-4-6" "https://github.com/octocat/Hello-World"