chore: sync workflows and streamline docs/comments

[spiegel-im-spiegel]

Summary

This PR aligns webinfo repository maintenance files and documentation style with goark/fetch, and updates module/tooling settings.

CI / workflow updates

• Replace legacy .github/workflows/lint.yml with .github/workflows/ci.yml
• Add .github/workflows/codeql.yml
• Use golang/govulncheck-action@v1 in CI

Taskfile and module updates

• Align Taskfile.yml structure with fetch
• Add dedicated govulncheck task using go run golang.org/x/vuln/cmd/govulncheck@latest
• Update dependencies to latest available versions
• Set module minimum Go version to 1.25.10
• Add toolchain go1.26.3

Test stability

• Accept .pjpeg extension in JPEG temporary-file test to absorb MIME extension variance across environments

Documentation and comments

• Rewrite README.md in concise fetch-style format
• Rewrite .github/copilot-instructions.md in concise fetch-style format
• Simplify verbose function comments in fetch.go and webinfo.go
• Update Go file copyright headers from 2025 to 2025-2026

Validation

• task prepare
• task test (go test + golangci-lint)