chore: add Codex and agent workflow foundation

docs/WORKFLOW.md

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+# Workflow
+
+## Branch policy
+
+Protected branches:
+- main
+- develop
+
+Normal task flow:
+1. checkout develop
+2. create feature/<task-name>
+3. implement the task
+4. commit changes
+5. merge feature branch into develop
+6. delete feature branch when appropriate
+
+## When to use an umbrella branch
+
+Use an umbrella branch only for a large coordinated effort that spans multiple dependent feature branches.
+
+If used, document:
+- why it exists
+- its exact name
+- which child branches merge into it
+- when it is expected to merge back into develop
+
+## Task discipline
+
+Each task should:
+- have one clear goal
+- have one task branch
+- end with a commit and merge target confirmation
+- not silently include unrelated cleanup