Add movetz and stallent as Swift SDK maintainers

[pcarleton]

Adding Maksym Mova (@movetz, MacPaw) and Stephen Tallent (@stallent, Mercury.io) as new maintainers for the Swift SDK.

Context

As discussed in the SDK working group and Discord, the Swift SDK has been without active maintenance since September 2025. Both MacPaw and Mercury.io have volunteered to take over co-maintenance:

• MacPaw (movetz): Maintains the Swift OpenAI library (3k stars), has been using MCP in production for 6+ months
• Mercury.io (stallent): Team of ~20 Swift developers, familiar with the codebase

Access Structure

This PR sets up a tiered access model (following the pattern used by go-sdk and rust-sdk):

Role	Permission	How to get it
Maintainers (mattt, movetz, stallent)	admin	Listed explicitly in repoAccess.ts
Contributors	push	Added to swift-sdk team in users.ts

Process for Adding New Contributors

When movetz/stallent want to add a new contributor to the Swift SDK:

1. The new contributor (or a maintainer on their behalf) opens a PR to this repo (modelcontextprotocol/access)
2. Edit src/config/users.ts to add the contributor to the swift-sdk team:

   {
     github: 'new-contributor-username',
     memberOf: ['swift-sdk'],
   },

3. movetz or stallent reviews and comments with approval on the PR
4. An access repo admin merges the PR
5. Once merged, the contributor automatically gets push access to swift-sdk

Note: movetz/stallent cannot directly merge PRs to this repo - they need to request a merge from one of the access repo admins after reviewing.