docs: add Salt to partners list

[0000F8]

Closes #2266.

Salt is an end-to-end encrypted chat where humans and AI agents are equal contacts. Every public Salt agent publishes a signed A2A AgentCard (e.g. https://saltapp.ai/api/.well-known/agent-card.json for Salt itself) and a did:web identity, so other A2A-aware tooling can discover and identify Salt agents. Agents on Salt message, ask a human with tappable buttons, invoice and get paid, inside an end-to-end encrypted chat.

One-line addition to docs/partners.md, alphabetically ordered, following the existing format.

🤖 Generated with Claude Code

https://claude.ai/code/session_01YQeHtC1zW2DtEs3qk5PvtK

[msampathkumar]

URL is reachable via a2a cli tool

$ a2a card get https://saltapp.ai/
Name:         Salt
Description:  Chat for the Human + AI Economy: end-to-end encrypted messaging, in-chat crypto payments, and a directory of AI agents humans and other agents can talk to, pay and hire. This card describes Salt itself, the directory -- meeting Salt starts with its concierge.
Version:      0.129.0
Interfaces:
  SALT_CHAT    https://saltapp.ai/agents/salt
Streaming:    false
Skills:
  cap-builds-plugins-for-your-chats Builds plugins for your chats
  cap-reminds-you-and-follows-up Reminds you and follows up
  cap-remembers-what-matters Remembers what matters
  cap-acts-for-you-with-permission Acts for you, with permission
  cap-watches-rooms-for-you Watches rooms for you

[msampathkumar]

@0000F8 - Thank you for the PR. Please note that, we do regular cleanup of broken url. If you ever change your domain or adjust it, please do raise a follow-up PR.

Also you can always reach out to us our a2a discord channel.