ci: refresh coverage badge on main events

[roark47]

Summary

• run CI on every push to main so coverage badge updates after docs-only or workflow-only merges too
• add workflow_dispatch so maintainers can manually refresh the coverage badge
• keep badge artifact upload and badges-branch write job restricted to main push/manual events, not PRs

Verification

• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/ci.yml")'
• git diff --check
• GOCACHE=/private/tmp/skill-up-gocache make verify
• GOCACHE=/private/tmp/skill-up-gocache make test