feat: add PandaDoc integration

[pranavrajs]

Adds the PandaDoc integration (Others). Look up contacts and check the status, recipients, and signing progress of quotes, proposals, and contracts in PandaDoc.

Tools

• Find Contact by Email (find_contact_by_email, GET)
• List Documents for Contact (list_documents_for_contact, GET)
• Search Documents (search_documents, GET)
• Get Document Status (get_document_status, GET)
• Get Document Details (get_document_details, GET)

Testing

• The manifest passes the validator ("valid": true).
• Every template was rendered locally with Liquid 5.4.0 against sample responses built from PandaDoc'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

• PandaDoc's auth docs call API keys a legacy method and say new integrations should use OAuth 2.0. That is a recommendation rather than a ban, but worth a decision before listing.
• Production API keys require approval through PandaDoc sales.
• Signing links, document content and field values are left out of the output.

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.

[pranavrajs]

Closing: no new integrations are being added in this round. The branch is kept in case this is wanted later.