fix(sdk): export A2A communicate subpaths

[miyaontherelay]

Summary

• export the existing A2A communicate modules from @agent-relay/sdk
• expose a2a-types, a2a-server, a2a-transport, and a2a-bridge as supported SDK subpaths
• unblock downstream consumers like Sage from using A2A support without local imports or copied code

Why

The A2A implementation already exists in Relay source, but the SDK package export surface does not expose it. Downstream consumers hit runtime/module resolution failures like:

• Missing "./communicate/a2a-transport" specifier in "@agent-relay/sdk" package

This fixes the actual upstream package boundary instead of papering over it downstream.

Validation

• attempted clean-worktree SDK build validation
• clean worktree surfaced a broader existing workspace/local-package resolution issue around @agent-relay/config, unrelated to this export change
• the export fix itself is focused and isolated to packages/sdk/package.json

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 1 additional finding.