README: add a "Community Integrations" section for third-party deployment repos

@denelon is already working on this.

Since Jun 3, 2026.

## Summary

Propose adding a new `## Community Integrations` section to MXC's README to give third-party projects that ship ready-to-use MXC configurations a discoverability surface. Filed as the feature-request issue per CONTRIBUTING.md before opening a PR.

## Motivation

MXC's value increases when projects beyond Microsoft are shipping production-validated configurations against it. Today there's no place in the README for an operator browsing the project to discover that those configurations exist — the existing tables cover Platforms, Schema Versions, and internal Documentation only.

We've built [`enclawed/mxc-enclawed-sandbox`](https://github.com/enclawed/mxc-enclawed-sandbox), a deployment repo shipping JSON policy configs for Bubblewrap / LXC / ProcessContainer / Seatbelt that drop [Enclawed](https://github.com/enclawed/enclawed-oss) (a classification-gated AI agent gateway) into each backend. Live-validated against MXC HEAD on Bubblewrap — transcript at [`mxc-validation.txt`](https://github.com/enclawed/mxc-enclawed-sandbox/blob/main/mxc-validation.txt) shows `enclawed 1.0.1` booting inside an MXC-managed sandbox with policy enforcement applied.

## Proposed section

Insert near the end of the README (between Documentation and Contributing):

\`\`\`markdown
## Community Integrations

Third-party projects that ship ready-to-use MXC configurations against their own runtimes. Each link points at the partner-owned repo where the configs and host setup are maintained.

| Project | Repo | Notes |
|---|---|---|
| [Enclawed](https://enclawed.com/) | [enclawed/mxc-enclawed-sandbox](https://github.com/enclawed/mxc-enclawed-sandbox) | Classification-gated AI agent gateway with MCP-attested transport. JSON configs for Bubblewrap / LXC / ProcessContainer / Seatbelt; live-validated against MXC HEAD on Bubblewrap. |
\`\`\`

## Scope and maintenance

Single doc-only section, no schema or code touched. Inclusion criteria (suggested, open to maintainer preference):

- Apache-2.0, MIT, or similar permissive license on the integration repo.
- Configs validate against at least one stable schema version.
- A captured validation transcript (or equivalent CI artifact) showing the integration actually boots inside an MXC backend.

Microsoft retains the curation decision; the table grows by maintainer-approved PR, not by self-add.

## Next step

If the team is open to this section, I'll file the PR (forking, draft-first per the contribution flow). Happy to refine the row text, the inclusion criteria, or the placement before I send the PR.