Allow custom agent to be specified via full qualified file path

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

The CLI should allow users to specify their custom agent path fully

### Proposed solution

Currently, copilot only allows custom agents to be referenced by name:
`copilot --agent `

The agents must be in one of the supported locations:
`$repoRoot/.github/agents`
`~/.copilot/agents`

This is limiting and users should be able to specify the fully qualified path to the custom agent definition:
`copilot --agent /path/to/agent`

This should be additive, if the fully qualified path isn't set, it should still look in the default directory locations.

### Example prompts or workflows

_No response_

### Additional context

_No response_