feat: enrich --version output with Go runtime and platform info

[Congregalis]

Summary

• Extend version.String() to include go, os, and arch fields
• Keep existing Version/Commit/BuildTime ldflags injection points unchanged
• Add a dedicated unit test for version.String()
• Update CLI version-output test expectation to match the new format

Closes #2

Validation

• go test ./internal/version ./internal/cli -run 'TestStringIncludesGoRuntimeAndPlatform|TestRunVersionSkipsAPIKeyRequirement'
• go build -ldflags "-X transblog/internal/version.Version=v9.9.9 -X transblog/internal/version.Commit=$(git rev-parse --short HEAD) -X transblog/internal/version.BuildTime=2026-03-12T15:40:00Z" -o /tmp/transblog-issue2-bin ./cmd/transblog
• /tmp/transblog-issue2-bin --version

[Congregalis]

这个 PR 本身改动 OK（同 #6）：版本输出补 go/os/arch + 测试齐。

但目前 PR #5 与 PR #6 内容完全重复（diff 一样）。为了减少重复与后续维护成本，建议关闭本 PR，保留另一个（比如 #6）合并即可。

[Congregalis]

Closing in favor of #6 (same diff).