feat: add CLI version output

[suraciii]

Summary

• add pages --version using the module version recorded in Go build information
• report (devel) for source builds without a module version
• update the CLI contract, root usage, README, and issue template
• cover exact stdout and invalid arguments with deterministic tests

This change does not add Docker image publishing or an official image workflow.

Verification

• make ci
• go test -race -count=1 ./...
• Linux, macOS, and Windows builds and test compilation
• go install github.com/suraciii/pages@cbc7a3c followed by pages --version