docs(readme): Add value proposition summary and remove table of contents

[justindobbs]

• Add "In short:" section comparing TraceCore to DeepEval/Braintrust/Langfuse with focus on software-like guarantees vs traceability/scoring
• Highlight TraceCore's unique positioning: reproducible runs, hard budgets, binary pass/fail, public certified agents via CI
• Note lightweight, open-source, spec-first approach for reliability engineering and ops workflows
• Remove table of contents section with 16 internal navigation

Summary

• What problem does this PR solve?
• How does it solve it (major bullets)?

Testing

• python -m pytest
• python -m ruff check agent_bench
• Additional targeted tests (list):

Checklist

• Spec/docs updated (README, SPEC_FREEZE, changelog, etc., as needed)
• New tasks/tests added to registry + SPEC if applicable
• Roadmap items tracked in appropriate boards (if applicable)
• Security/privacy review (if touching telemetry, signing, or bundles)
• Verified CI status once pushed