Auto-generate workspace branch name from user prompt

## Feature Request

When creating a workspace, Orca should automatically generate the branch name from the user's prompt using the model configured in settings — no manual naming required.

**Use case:** User types a task description to create a workspace. Instead of manually entering a branch name, Orca generates one using the configured model.

**Proposed behavior:**
- Branch name generated from the user's prompt using the model set in user settings
- Format: conventional prefix + short kebab-case slug
- Valid prefixes: `feat/`, `fix/`, `chore/`, `docs/`, `refactor/`, `test/`, `ci/`, `perf/`, `style/`, `build/`
- Examples: `feat/add-user-auth`, `fix/null-pointer-crash`, `chore/update-deps`
- User can override the generated name before confirming