feat: add Trello integration

[pranavrajs]

Adds the Trello integration (Others). Search Trello cards, check their status and comments, and add comments to cards.

Tools

• Search Cards (search_cards, GET)
• Get Card (get_card, GET)
• List Card Comments (list_card_comments, GET)
• List Boards (list_boards, GET)
• List Board Cards (list_board_cards, GET)
• Add Card Comment (add_card_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 Trello'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.

Review notes

• API key and token are sent in Trello's documented Authorization: OAuth header, not in the URL.
• The search template expects {cards: [...]}, which differs from what Trello's OpenAPI spec describes.

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.