feat: register HttpAgent in CLI with --agent http

[abrichr]

Summary

• Registers http as a valid --agent type in all three CLI subcommands (mock, run, live)
• Adds --agent-endpoint argument for specifying the remote agent URL
• Validates that --agent-endpoint is provided when --agent http is used

Usage:

openadapt-evals run --agent http --agent-endpoint http://gpu-box:8080 --task notepad_1
openadapt-evals live --agent http --agent-endpoint http://gpu-box:8080 --task-ids notepad_1
openadapt-evals mock --agent http --agent-endpoint http://localhost:8080

Follows up on #118 which added the HttpAgent class.

Test plan

• openadapt-evals run --help shows http in agent types and --agent-endpoint
• openadapt-evals mock --agent http without --agent-endpoint prints clear error
• All 79 relevant tests pass (http_agent, rl_env, mock_adapter)

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