[AS-2597] feat: migrate Go tutorial to app-sdk

[pbc1017]

Summary

• migrate all Go imports and go.mod to github.com/channel-io/app-sdk/go v0.14.0
• switch documentation links to channel-io/app-sdk
• add complete Korean and Japanese Go tutorial and WAM guides
• remove platform-specific runtime wording and link localized Extension guides

Test Plan

• Go 1.25.12 build
• go test ./...
• WAM build
• Yarn format, lint, and typecheck
• Prettier and public-name audit

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Review details

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: a71f926a-60f2-49c0-9f03-6634a81493f7

📥 Commits

Reviewing files that changed from the base of the PR and between 5232fb3 and 1edfea0.

⛔ Files ignored due to path filters (1)

• go.sum is excluded by !**/*.sum

📒 Files selected for processing (13)

• README.ja.md
• README.ko.md
• README.md
• cmd/function_endpoint.go
• cmd/function_endpoint_test.go
• cmd/main.go
• go.mod
• internal/tutorial/app.go
• internal/tutorial/app_test.go
• internal/tutorial/native_message.go
• wam/README.ja.md
• wam/README.ko.md
• wam/README.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch codex/app-sdk

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