feat(config): add commands for worker model, scout model, and budget ceiling

[akougkas]

Problem

Three frequently needed configuration surfaces have no slash command:

1. Worker model (models.worker): The model used for dispatched workers. Currently only settable via PANCODE_WORKER_MODEL env var or pan_apply_config tool. The /preferences worker-model sub-panel exists but is display-only.

2. Scout model (models.scout): The model used for shadow scout agents. Same situation as worker model.

3. Budget ceiling (budget.ceiling): /budget displays the ceiling but cannot change it. Users must set PANCODE_BUDGET_CEILING env var or use the tool.

These are the most commonly adjusted parameters after mode and safety. Making users restart PanCode or ask the LLM to change these is a poor experience.

Audit Reference

AUDIT-ISSUES.md: E1, E2, E3

Objective

Add direct-action commands for the three most-used uncovered config surfaces.

Deliverables

• Add /worker-model [provider/model-id] command (or extend /models worker <ref>)
• Add /scout-model [provider/model-id] command (or extend /models scout <ref>)
• Make /budget <amount> set the ceiling when an argument is provided
• All three display current value when called without arguments
• Persist changes to settings.json
• Emit CONFIG_CHANGED events