MUL-6016: feat(task): retry exact Codex capacity failures (#6747)

* docs: record Codex capacity retry design

* docs: add Codex capacity retry implementation plan

* fix(taskfailure): recognize exact Codex capacity error

* feat(task): retry exact Codex capacity failures

* feat(server): configure Codex capacity retries

* docs: expose Codex capacity retry setting

* fix(task): align capacity retry comments

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Co-authored-by: cuijs <cuijs@explore.com>
Co-authored-by: Claude <noreply@anthropic.com>