Decouple agent sandbox boot from Data Machine

[chubes4]

Summary

• Removes Data Machine-specific workspace path, backend filter, workspace-adoption, PermissionHelper, plugin activation, and diagnostic probes from generated agent sandbox boot/probe code.
• Leaves Agents API runtime-principal authorization, default agents/chat execution, runtime task ability execution, and generic bundle-import hooks intact.
• Adds a generic mounted-runtime bootstrap context/action that mounted components can consume before WordPress lifecycle hooks run.

Fixes #842.

Paired upstream

• Data Machine Code self-configuration PR: Add mounted sandbox self-configuration Extra-Chill/data-machine-code#724
• The DMC PR consumes the generic bootstrap context and owns DMC-specific workspace path, backend, and adoption setup.

Verification

wp-codebox:

• npx tsx scripts/agent-sandbox-code-smoke.ts
• npx tsx scripts/agent-sandbox-workspace-root-smoke.ts
• npx tsx scripts/agent-task-run-runtime-components-smoke.ts
• npm run build

Data Machine Code paired PR:

• php tests/smoke-mounted-sandbox-bootstrap.php
• php tests/smoke-remote-workspace-backend-filter.php
• php -l inc/Runtime/MountedSandboxBootstrap.php
• php -l tests/smoke-mounted-sandbox-bootstrap.php

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (GPT-5.5)
• Used for: Implemented the Data Machine decoupling in the sandbox boot generator, added the generic mounted-runtime seam, updated targeted smoke coverage, and ran verification. Chris remains responsible for reviewing and testing the change before merge.