Add ship test marker file

[MarcStoecker]

Rationale

Verify the ship execution loop works end-to-end by completing a trivial task.

Scope

In scope:

• Create one text file

Out of scope:

• Everything else

Acceptance Criteria

• File .mill/ship/test-marker.txt exists with content ship works

Loop Contract

• Test command: test -f .mill/ship/test-marker.txt && grep -q "ship works" .mill/ship/test-marker.txt && echo PASS
• Success criteria: Command outputs PASS
• Stop conditions: 5 iterations max

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🤖 Claude