Fixes #324: Implement DesignIteration Agent Skill Example

[davideast]

Fixes #324

This PR implements the DesignIteration Agent Skill example as requested in issue #324.
It adds SKILL.md and README.md to packages/sdk/examples/design-iteration and links the new example in the SDK's README.md.

/label fleet-merge-ready
/milestone 1

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PR created automatically by Jules for task 6855512939550690510 started by @davideast

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⚠️ Closed by fleet-merge: batch conflict resolution dispatched (session 5860293050553752988).

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[jules-fleet]

🔄 Batch conflict resolution in progress

This PR is part of a batch being resolved together: #309, #332, #333, #334, #335, #336, #337, #338, #339, #341, #342, #340, #343
A Jules session has been dispatched to create a combined PR that resolves all conflicts.

Session: 5860293050553752988