chore: add Codex and agent workflow foundation

docs/MACHINE_SETUP.md

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+# Machine Setup
+
+## Goal
+
+Bring a new machine to a usable Codex development state with minimal local-only setup.
+
+## Local-only prerequisites
+
+Not stored in the repo:
+- Codex authentication
+- personal shell setup
+- secrets/API keys
+- local editor preferences
+
+## Repository bootstrap
+
+1. Clone the repository
+2. Checkout `develop`
+3. Read:
+   - AGENTS.md
+   - docs/WORKFLOW.md
+   - docs/TASK_TEMPLATE.md
+4. Ensure Codex can see:
+   - AGENTS.md
+   - .codex/config.toml
+   - .codex/agents/*
+   - .agents/skills/*
+
+## Validation checklist
+
+A machine is considered ready when:
+- repo is cloned
+- correct branch is checked out
+- Codex authentication is working
+- AGENTS.md is present at repo root
+- .codex/config.toml is present
+- task branches can be created from develop