fix: soften downstream experiment trigger

[Pigbibi]

Summary

• switch the downstream experiment trigger to gh label/issue editing
• keep the label trigger non-fatal so the monthly planner no longer fails on downstream permission quirks

Testing

• python3 -m unittest tests/test_experiment_validation_workflow_config.py tests/test_monthly_optimization_planner_workflow_config.py
• uvx ruff check scripts tests
• ruby -e 'require "yaml"; [".github/workflows/experiment_validation.yml", ".github/workflows/monthly_optimization_planner.yml"].each { |p| YAML.load_file(p); puts "parsed #{p}" }'