Feature Request: Directory-specific default agent configuration

### Checks

- [x] I have searched [github.com/aws/amazon-q-developer-cli/issues](https://github.com/aws/amazon-q-developer-cli/issues?q=) and there are no duplicates of my issue
- [x] I have run `q doctor` in the affected terminal session
- [x] I have run `q restart` and replicated the issue again

### Operating system

macOS 15.6.1 (24G90)

### Expected behaviour

Currently, Amazon Q Developer CLI only supports global default agent configuration via 'q settings chat.defaultAgent'. This requires users to manually switch default agents when working on different projects.

#### Requested Feature:
- Support for directory-specific default agent configuration
- Ability to set default agent per project/directory (e.g., .amazonq/config.json with defaultAgent setting)
- Automatic agent selection based on current working directory

#### Use Case:
- Different projects require different specialized agents
- GenU maintenance projects need genu-maintenance agent by default
- Other projects might need different domain-specific agents

This would improve developer workflow efficiency when working with multiple projects requiring different agents."

### Actual behaviour

- Manual agent specification with --agent flag for each command
- Frequent global default agent switching

### Steps to reproduce

_No response_

### Environment

```yaml

[q-details]
version = "1.16.1"
variant = "full"

[system-info]
os = "macOS 15.6.1 (24G90)"
chip = "Apple M1 Pro"
total-cores = 8
memory = "16.00 GB"

```