chore: update line-bot-sdk-go to v8.22.0 and rewrite README - #31

Summary

Brings the template up to the latest LINE Messaging API SDK, modernizes main.go for the Go version that comes with it, and gives the README a pass.

SDK / toolchain

• github.com/line/line-bot-sdk-go/v8: v8.10.0 → v8.22.0
• go.mod go directive: 1.23 → 1.25 — v8.22.0 declares go 1.25, so this is required, not optional
• // +heroku goVersion pragma updated from go1.21 to go1.25 to match
• CI: actions/setup-go@v4 → @v5, pinned Go 1.21 → 1.25 (the old pin could no longer build the new SDK)

The SDK bump itself needed no source changes — main.go compiled against v8.22.0 unmodified.

Go modernization

Checked main.go against the Modern Go Guidelines for Go 1.25; two applied:

• Method-aware ServeMux pattern (Go 1.22+): the webhook is now registered as "POST /callback" rather than "/callback". LINE only ever POSTs to the webhook, so anything else now gets a 405 from the mux instead of falling into the handler and failing signature parsing.
• cmp.Or: the PORT fallback collapses from a four-line empty-string check to cmp.Or(os.Getenv("PORT"), "5000").

README

• Fixed typos (devloper, Rener, Hoe to)
• Added What it does, Requirements, and an Environment variables table (ChannelSecret, ChannelAccessToken, optional PORT)
• Added a Running locally section with the go run + ngrok tunnel flow, which was missing entirely
• Added Making it your own — where the event loop is and how to extend it (other message types, richer reply messages, non-message events, push vs. reply)
• Tightened the Render / Heroku deploy steps and the webhook-URL step, including the reminder to disable OA auto-reply
• Switched to consistent ATX headings throughout

Test plan

• go build ./...
• go vet ./...
• gofmt -l . clean
• go test ./... (no test files in this repo)