docs: fix hardware provider framing, remove em dashes

[imran-siddique]

Summary

• Remove all em dashes from the README (title, Why TEE table, quickstart sentence) per style rules
• Fix the auto-detect config comment: correct provider order (sev-snp -> tdx -> tpm -> software) and clarify opaque is explicit opt-in via OPAQUE_ATTESTATION_URL, not in the auto-detect chain
• Change opaque assurance level from Highest to High (explicit opt-in) to accurately reflect its status
• Add gpu-cc row (NVIDIA H100/H200/Blackwell in CC mode, attested via NRAS) to the hardware providers table
• Add Attestation Service column to the hardware providers table for clarity

Test plan

• Render README on GitHub and confirm no em dashes appear
• Verify hardware providers table renders correctly with 5 rows and 4 columns
• Confirm auto-detect comment in the config snippet matches actual runtime behavior

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