fix(agent): recover from Codex app-server write failures

[NikitaMGrimm]

Summary

• type app-server stdin/write failures separately from programming errors
• retire dead Codex sessions after turn-start and approval-response write failures
• make steer/interrupt shutdown races non-fatal so the next turn can start a fresh app-server
• preserve the upstream commits and original authorship from fix(agent): return failed turns when Codex app-server exits during RPC writes NousResearch/hermes-agent#83129

Validation

• 7 newly added transport/write-failure tests pass
• combined app-server runtime/session suite: 64 pass; 2 pre-existing container-only HOME contract failures unrelated to this diff
• git diff --check passes