docs(browser): clarify snippet persistence

[MagMueller]

Summary

• clarify that only session persists between browser_execute calls
• direct reusable variables and functions into workspace modules imported by each snippet

Validation

• git diff --check

Closes #132

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Summary by cubic

Clarify snippet persistence in browser_execute docs: only session persists across calls; variables and functions do not. Recommend placing reusable code in workspace files and importing it from each snippet.

^{Written for commit 2d2644f. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 1 file

_{Re-trigger cubic}