feat: add Todoist integration

[pranavrajs]

Adds the Todoist integration (Others). Search Todoist tasks, check their details and comments, and add comments to tasks.

Tools

• Search Tasks (search_tasks, GET)
• Get Task (get_task, GET)
• List Task Comments (list_task_comments, GET)
• List Projects (list_projects, GET)
• List Project Tasks (list_project_tasks, GET)
• Add Task Comment (add_task_comment, POST)

Testing

• The manifest passes the validator ("valid": true).
• Every template was rendered locally with Liquid 5.4.0 against sample responses built from Todoist's API docs.
• Not run against the live API. No credentials were available, so tool-tester.rb has not been run with a real key. This needs doing before merge.
• No logo is included.

The root README.md row and .env.example entry for this integration are in a separate PR, so the per-integration PRs do not conflict with each other.