Unify Agent config generation between TUI dev mode and pyinfra deploy

## Summary

Extract Agent configuration generation (agent.toml) into a shared module.

## Current Duplication

- `context.py:configure_agent()` — sed-based modifications
- `deploy/cores/agent/` — Jinja2 template
- Accelerator plugin selection logic duplicated (CUDA_MOCK, ROCM_MOCK vs AcceleratorType enum)

## Complexity: SIMPLE

Agent config is relatively straightforward — ports, paths, and accelerator plugin selection.

## Approach

Create `src/ai/backend/install/config_gen/agent.py` with config dict generation functions.

JIRA Issue: BA-5568