Add a Codex on AgentCore Runtime with EFS sample under 12-coding-agents

Currently `06-workshops/01-AgentCore-runtime/12-coding-agents` contains two Claude Code samples (`01-claude-code-with-s3-files`, `02-claude-code-with-efs`). There is no sample covering the OpenAI Codex SDK on AgentCore Runtime.

Codex is not a drop-in substitution for Claude Code in the existing EFS sample. The setup differs in ways that are easy to get wrong and are worth documenting:

- Codex reads its provider configuration from `$CODEX_HOME/config.toml` (`model_provider = "amazon-bedrock"`). There is no equivalent of the `CLAUDE_CODE_USE_BEDROCK=1` env var.
- The GPT-5.6 family (`openai.gpt-5.6-sol` and siblings) is served through the `bedrock-mantle` endpoint, so the execution role needs `bedrock-mantle:CreateInference` and `bedrock-mantle:CallWithBearerToken` rather than only `bedrock:InvokeModel`.
- Because `CODEX_HOME` can live on EFS, a Codex *thread* becomes persistent state rather than session state, so a conversation can be resumed from a brand new runtime session. This makes a nice, concrete demonstration of why EFS matters and complements what `02-claude-code-with-efs` already shows about shared skills.
- Codex refuses to run outside a git repository unless `skipGitRepoCheck` is set, so the workspace needs initializing on first boot.

### Proposal

Add `03-codex-with-efs` as a sibling to `02-claude-code-with-efs`, mirroring its file layout and scripts (`setup.sh`, `deploy.py`, `update.py`, `invoke.py`, `exec_cmd.py`, `cleanup.py`, `cfn-vpc.yaml`, `Dockerfile`, `server.js`) so the two can be read side by side and the Codex-specific differences stand out.

I have this implemented and tested and will open a PR referencing this issue.

Compare 06-workshops/01-AgentCore-runtime/12-coding-agents/02-claude-code-with-efs with the proposed 03-codex-with-efs sibling, then read the named setup.sh, deploy.py, update.py, invoke.py, exec_cmd.py, cleanup.py, cfn-vpc.yaml, Dockerfile, and server.js files. Follow the sample's deployment and invocation flow; done means the Codex EFS sample is present, tested, and demonstrates persistent threads alongside the existing Claude Code sample.

Written by the indexing model from the issue text.