Auto-allow permissions on session start

### Describe the feature or problem you'd like to solve

Add a persistent setting to automatically run /allow-all when starting a new session.

### Proposed solution

Suggested implementation:
- Add a setting like `permissions.auto_allow_all: true` (configurable via /settings)
- Or add a startup preference in ~/.copilot/config

### Example prompts or workflows

I work in trusted directories and need to run /allow-all at the start of every session. Having to manually enable permissions each time creates friction in the workflow.

### Additional context

Same as Claude's bypass permissions toggle.

Start by tracing the existing /allow-all command and the /settings flow. Compare the proposed permissions.auto_allow_all setting with the startup preference in ~/.copilot/config, then determine how a new session reads persisted settings. Done means a documented, configurable preference automatically applies the intended permission behavior when a session starts.

Written by the indexing model from the issue text.