ci: reduce GitHub Actions usage

[M09Ic]

Summary

• remove the nightly release workflow while preserving existing releases and tags
• consolidate pull request CI from 18 jobs to 6 and eliminate duplicate checks/builds/artifact uploads
• move repeated race stress tests to the weekly scanner regression workflow
• group Dependabot updates and add an act configuration for stable local validation
• stabilize PTY wrapper tests affected by instantaneous process-exit timing

Validation

• actionlint
• workflow YAML parsing
• go mod tidy -diff
• git diff --check
• go test -count=1 ./archtest ./agent/tmux ./core/deps
• full Linux race test command and generated-resource check in the act test container