Revert MCP SDK upgrade to fix protocol version mismatch (#760)

## Summary
Reverts #759 to restore MCP server functionality.

## Problem
The MCP SDK upgrade caused a **protocol version mismatch**:
- Claude Code sends: `MCP-Protocol-Version: 2025-11-25`
- Our server (via `agents@0.2.23`) only supports: `2025-03-26,
2025-06-18`

The SDK upgrade alone doesn't help because protocol version support
comes from the `agents` package, not the MCP SDK.

## Root Cause
Supporting protocol `2025-11-25` requires:
- `agents@0.3.5+` (uses MCP SDK 1.25.2 with the newer protocol)
- `ai@6.x` (required by agents 0.3.x)
- This is a significant migration with breaking changes

## Next Steps
- AI SDK 6 migration tracked separately in #756
- This revert restores service for users on older Claude Code versions

Fixes #756

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Author: dcramer

pnpm-lock.yaml

@@ -10,7 +10,7 @@ catalog:
   "@cloudflare/vitest-pool-workers": ^0.8.47
   "@cloudflare/workers-oauth-provider": ^0.0.12
   "@cloudflare/workers-types": ^4.20251014.0
-  "@modelcontextprotocol/sdk": ^1.25.3
+  "@modelcontextprotocol/sdk": ^1.21.0
   "@radix-ui/react-accordion": ^1.2.11
   "@radix-ui/react-slot": ^1.2.3
   "@sentry/cloudflare": 10.35.0