[codex] Upgrade Go and polish README

[maxwu]

Summary

• Upgrade the module and mise toolchain declaration to Go 1.26.
• Refresh the README with accurate API usage, install instructions, and development guidance.
• Align the local Makefile test target with CI and add repo-local agent guidance files.

Validation

• GOCACHE=/private/tmp/chainer-go-build mise exec -- make test

Notes

• mise.toml is intentionally included even though the local global gitignore ignores it, because this repo now documents mise as the local toolchain manager.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (1ff8803) to head (02e15c6).

Additional details and impacted files

@@            Coverage Diff            @@
##              main        #5   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines           11         9    -2     
=========================================
- Hits            11         9    -2     

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

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