RFD to rework Rust SDK, modeled on sacp (#377)

* feat: RFD to adopt `sacp` as the Rust SDK

* chore: run prettier

* chore: fix typo

* docs: clarify that sacp will be imported into agent-client-protocol

- Update 'What we propose' to show sacp → agent-client-protocol-* rename table
- Update crate structure section with new naming
- Rewrite migration path to clarify sacp deprecation (not the other way around)
- Change code examples from sacp:: to acp:: (use agent_client_protocol as acp)
- Update sacp-rmcp references to agent-client-protocol-rmcp
- Clarify conductor is implementation of proxy chains RFD
- Remove links to symposium-acp source code

* Use v1 instead of v2

* Update website

---------

Co-authored-by: Ben Brandt <[email protected]>

Author: nikomatsakis

docs/docs.json

@@ -116,7 +116,8 @@
               "rfds/mcp-over-acp",
               "rfds/session-usage",
               "rfds/acp-agent-registry",
-              "rfds/auth-methods"
+              "rfds/auth-methods",
+              "rfds/rust-sdk-v1"
             ]
           },
           { "group": "Preview", "pages": ["rfds/session-config-options"] },