docs: add negotiated structured text formats RFD - #2315

Summary

This RFD proposes an optional, backward-compatible negotiation mechanism for structured text formats in ACP agent messages.

• Markdown remains the existing default.
• Clients and agents may negotiate text/plain, text/markdown, or text/org.
• Each text content block carries a stable format/profile across streaming chunks and replay.
• A Portable Org profile covers headlines, paragraphs, lists, emphasis, links, tables, and safe static blocks.
• Experimental deployments can begin under namespaced _meta capabilities before schema changes.

Related discussion

This expands the response-format discussion in ACP Discussion #2147 with negotiated text output and native Org-mode rendering.

Scope

This pull request adds the RFD document only. It does not change the wire schema or SDKs yet; those are proposed as later phases after review.

Questions for reviewers

• Should format/profile fields be added directly to TextContent, or begin as namespaced _meta?
• Should negotiation be prompt-level, session-level, per-content-block, or support all three?
• Is text/org plus a portable profile the right label, and how should any shared MCP content-block change be coordinated?