Add custom UI for golden-agent

[declan-scale]

Summary

• Adds a new /custom page in agentex-ui that targets the golden-agent deployment
• Introduces a config panel, chat panel, prompt input, and tag input components for driving custom agent runs
• Wires the page through AgentexProvider and reuses useAgents / ResizableSidebar infrastructure

Test plan

• npm run typecheck in agentex-ui/
• npm run lint in agentex-ui/
• Run ./dev.sh, deploy golden-agent, navigate to /custom, and verify config panel, prompt submission, and chat output
• Confirm config panel locks after a task is created and unlocks on reset

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