Correct issue and PR notification rendering

[Nanaloveyuki]

Summary

• Populate the common action template field for all webhook event types.
• Link Issue comments to their parent Issue and fix the malformed PR update emoji.
• Remove the redundant PR separator that appeared when the PR body was empty.

Validation

• go test ./internal/handler ./internal/template ./internal/confign- go build ./...n
  Related to Message displaying bugs discovered during daily use #11. This intentionally does not cover image and code-block rendering, which requires a separate Feishu card/content conversion design.

[hnrobert]

sugoi