Amazon Q: Add configuration option to save custom prompts in workspace directory

## Problem

Currently, Amazon Q custom prompts are saved globally in `~/.aws/amazonq/prompts`. This creates limitations for:
- Project-specific prompt management
- Team collaboration through version control
- Environment isolation between different projects

## Expected behavior

Add a configuration option `amazonQ.promptsLocation` to allow users to choose where custom prompts are stored:
- `"global"` (default): Current behavior using `~/.aws/amazonq/prompts`
- `"workspace"`: Save prompts in workspace root directory (e.g., `.amazonq/prompts`)

## Use cases

1. **Project-specific prompts**: Different projects may require tailored prompts for their specific domain, coding standards, or requirements
2. **Team collaboration**: Share custom prompts with team members through version control
3. **Environment isolation**: Keep development, staging, and production prompts separate

## Related

This addresses similar needs as the closed issue #6415 but focuses specifically on the storage location configuration rather than workspace-local prompt configuration.

No files or tests are named in the issue. Start by locating the existing Amazon Q custom-prompt storage and configuration entry points, then verify the global default and workspace behavior. Done means prompts use ~/.aws/amazonq/prompts by default and .amazonq/prompts under the workspace when amazonQ.promptsLocation is set to "workspace".

Written by the indexing model from the issue text.