chore(deps): bump spx to 2.0.4

[nighca]

Summary

• Bump XBuilder's SPX runtime version from 2.0.3 to 2.0.4
• Refresh SPX Go module dependencies in tools/ai, tools/spxls, and tools/ispx
• Document that tools/spxls has no packages to test when running the SPX upgrader checklist

Release notes

https://github.com/goplus/spx/releases/tag/v2.0.4

Validation

• gh release view --repo goplus/spx v2.0.4
• bash spx-gui/install-spx.sh
• bash build-wasm.sh in spx-gui/
• npm ci in spx-gui/
• npm run lint in spx-gui/
• npm run test -- --run in spx-gui/ (73 files, 703 tests)
• go test ./... in tools/ai/
• go test ./... in tools/ispx/
• go test ./... in tools/spxls/ reported no packages to test, which is expected for this module

[gemini-code-assist]

Code Review

This pull request upgrades the spx dependency from version 2.0.3 to 2.0.4 across the repository, including configuration files, installation scripts, and Go module dependencies (tools/ai, tools/ispx, and tools/spxls). Additionally, it updates the documentation in .github/agents/spx-upgrader.md to clarify that warning messages regarding unmatched packages during testing of tools/spxls/ are acceptable. There are no review comments, and I have no feedback to provide.

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