feat(agents): add first-class Codex CLI runtime

[jdchawla29]

Summary

• add a first-class Codex CLI adapter on the registered agent contract
• isolate CODEX_HOME and generated CLI configuration per agent process
• resolve managed CLI agent runtimes through the shared registry
• harden Claude and Codex 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 2 of 5.

Depends on #624.
Next layer: codex/runtime-inference-routing.

[jdchawla29]

Folded into #624 by fast-forwarding #624 to this PR's exact head. #626 now bases directly on #624, so the final Claude refactor and Codex adapter are reviewed together.