feat(agents): add first-class Claude Code and Codex CLI runtimes

[jdchawla29]

Summary

• establish the registered agent configuration and load/dump contract
• make agent execution ownership explicit in the rollout loop
• replace the Claude SDK implementation with a first-class Claude Code CLI adapter
• add Codex on the shared CLI adapter contract with isolated process configuration
• harden prompt transport and disable Claude compaction for gateway-backed runs

Validation

• 726 focused agent, eval, environment, capability, client, and CLI tests passed
• repository CI formatting check passed
• repository CI Ruff check passed
• repository CI full-extra type check passed

Stack

Layer 1 of 4. Base: main.

Next layer: #626.