feat: Add Elixir community SDK (ag_ui_ex v0.1.0)

[peguesj]

Summary

• Adds ag_ui_ex as a community Elixir SDK under sdks/community/elixir/
• Full source mirror of peguesj/ag_ui_ex v0.1.0
• Adds docs/sdk/elixir/overview.mdx with installation, quick start examples, and module reference
• Inserts Elixir row (Supported) in README.md SDKs table (after Ruby)
• Inserts Ruby (previously missing) + Elixir rows in docs/introduction.mdx SDKs table

What's in the SDK

ag_ui_ex is a complete Elixir implementation of the AG-UI protocol:

• 30+ typed event structs — All AG-UI event types as Elixir structs with validation
• SSE transport — Plug-compatible Server-Sent Events for Phoenix/Plug applications
• Channel transport — Phoenix Channel macros for WebSocket-based event streaming
• Middleware pipeline — Composable event processing with built-in middleware
• State management — GenServer with snapshot/delta pattern and full RFC 6902 JSON Patch
• HTTP client — GenServer for consuming AG-UI event streams over SSE
• Encoder — SSE and JSON encoding/decoding with camelCase <-> snake_case conversion

Package

• Hex.pm: https://hex.pm/packages/ag_ui_ex (v0.1.0)
• Source: https://github.com/peguesj/ag_ui_ex
• License: MIT
• Elixir: ~> 1.15 / OTP ~> 26

Test plan

• Verify sdks/community/elixir/ contains full SDK source
• Verify docs/sdk/elixir/overview.mdx renders correctly in docs site
• Verify Elixir row appears in README.md SDKs table
• Verify Ruby + Elixir rows appear in docs/introduction.mdx SDKs table
• Run mix test in sdks/community/elixir/ (all tests pass on Elixir 1.15+)

Fixes #1291

[vercel]

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