Allow approval comments to include reasons

[CAOShurong]

Summary

• add an opt-in allow-comment-reasons input
• when enabled, read the approval or denial decision from the first line and allow explanatory text on later lines
• keep the input disabled by default so existing v1 exact-comment behavior is unchanged

Closes #21.

Verification

• go test -count=20 ./...
• go vet ./...
• go build ./...
• golangci-lint v2.10.1 run -v (0 issues)
• parsed action.yaml and verified the new default is the string false
• ran the compiled action against a local fake GitHub API for three observable paths: approval with a reason, denial with a reason, and the default compatibility path where a reason-bearing comment remains pending until an exact approval comment arrives
• public fork CI passed the upstream Linux Docker build, tests, and v2.10.1 lint on exact head 6bd8130

OpenAI Codex assisted with implementation and test execution. I reviewed the change and the reported results before submission.