fix(codex_sdk): keep provider credentials out of Codex run artifacts

[Glen-SP]

Summary

Codex captures its own environment into shell_snapshots/*.sh, and on relay
provider routes _client_process_env puts the provider key into that
environment, so the key was written verbatim into run artifacts (#84).

features.shell_snapshot=false (#196) is applied on every auth path, but
openai-codex 0.147.0 accepts the flag and still writes snapshots. The effective
fix: once start_relay holds the real key for the upstream hop, the child's
copy is replaced with a placeholder. Codex authenticates only against the relay
(requires_openai_auth=false), so it never needed the credential. The flag is
kept as defence in depth.

Refs #84

Scope

• Affected modules or public contracts:
  praxist/plugins/agent_runtimes/codex_sdk/adapter.py and its unit tests. No
  public contract change.
• Compatibility considerations: relay routes (10 of 11 providers) are covered.
  The direct openai route still needs the real key in the Codex environment
  and is not changed here. Existing snapshots in old run directories are not
  scrubbed.
• Task-agnostic rationale: credential handling in the runtime adapter is shared
  by every task and provider route.

Verification

Check	Before	After
Flag present in Codex argv	yes	yes
Snapshots written	yes	yes (flag ignored)
Snapshots containing the real key	all	0
Run artifacts containing the real key	≥ 2 per run	0 across 8 runs

• ruff check / ruff format --check — clean (461 files)
• pyrefly check — 0 errors
• pytest — 3365 passed, 7 skipped
• run_test_coverage.py unit --fail-under 90 --fail-under-statements 95 — exit 0
  (95.03% statements); integration — exit 0
• The new test fails when the placeholder assignment is reverted.
• Real tasks on this branch: 3 MLE-bench runs (2 gold) and 4 reference tasks,
  all succeeded.

Could not be run: the direct openai route, which this change does not cover.

Checklist

• The change is focused and does not include unrelated generated files.
• Tests cover the affected behavior.
• Documentation, templates, examples, and skills were updated where needed.
• No credentials, private task data, or research-run artifacts are included.
• New dependencies and copied assets include their source and license terms.
• I have read .github/CONTRIBUTING.md and the contribution terms in LICENSE.md.