fix(opencode): grant MCP tool permissions in subagent sessions

[ZK-Andy]

Issue for this PR

Closes #16491

Type of change

• Bug fix

What does this PR do?

Subagents spawned via the Task tool can now execute MCP tools. Previously, MCP tools appeared in the subagent tool registry but permission checks failed because they were not granted in the session permission array.

This fix retrieves available MCP tools from the MCP service and:

1. Adds them to the session permission array with allow action
2. Includes them in the tools object passed to SessionPrompt.prompt() (default disabled)

How did you verify your code works?

• TypeScript typecheck passes (\ sgo --noEmit)
• The test suite layer includes \MCP.defaultLayer\ to satisfy service requirements
• Without MCP configured, \mcp.tools()\ returns empty object, making the new code a no-op

Screenshots / recordings

N/A

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[ZK-Andy]

@simonklee @kitlangton @thdxr @rekram1-node I realize everyone is busy, but this fix is important for workflows that rely on MCP tools in multi-step agent tasks. Without it, subagents can't access those tools at all. Any review would be much appreciated. Thanks!