docs(blog): add "Verify the description, not the effect" post

[hubyrod]

Summary

Adds a new blog post, www/blog/side_effects.md, that answers how Skipper's closed-loop coding agent verifies side effects it can't cheaply or totally observe — by checking the effect's description against a fixed contract rather than the effect itself.

The post covers:

• Why side effects break the naive verify-loop pitch (checks must be cheap and total)
• The compiler analogy: verify the call against its signature without performing the effect
• Effects as typed values / externals, typed-double sandboxing, and the contract as the unit of verification
• An honest section on what the loop does not claim (delivery, idempotency, retries)

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