feat: link Tobiko Cloud plans from GitHub CICD bot

[mday-io]

Description

Adds optional Tobiko Cloud plan-link comments to the GitHub CICD bot.

• Posts/updates a PR environment plan comment when TCLOUD_URL or TCLOUD_BASE_URL is configured.
• Posts/updates a production deploy plan comment before production apply so failed applies can still expose the generated Tobiko Cloud plan link.
• Keeps non-Tobiko workflows unchanged when those environment variables are unset.
• Updates existing marker comments in place and clears stale marker comments on no-op runs.
• Documents the optional workflow environment variables in the GitHub CICD bot docs.

Test Plan

• make style
• make fast-test
• pytest tests/integrations/github/cicd/test_github_controller.py -q
• pytest tests/integrations/github/cicd/test_github_commands.py tests/integrations/github/cicd/test_integration.py -q
• ruff check sqlmesh/integrations/github/cicd/controller.py tests/integrations/github/cicd/test_github_controller.py
• ruff format --check sqlmesh/integrations/github/cicd/controller.py tests/integrations/github/cicd/test_github_controller.py

Checklist

• I have run make style and fixed any issues
• I have added tests for my changes (if applicable)
• All existing tests pass (make fast-test)
• My commits are signed off (git commit -s) per the DCO