docs: add Open Knowledge to clients list

[amikofalvy]

Adds Open Knowledge to the Editors and IDEs section (alphabetically, between Obsidian and Pulsar).

What is Open Knowledge?

Open Knowledge is an open-source, local-first Markdown knowledge base and editor (an "AI-native markdown IDE") with 3.4k+ stars and 220+ forks on GitHub. It ships as a desktop app for macOS, Windows, and Linux, and as a CLI (@inkeep/open-knowledge on npm).

ACP support is native and first-party

• Agent threads run in a side panel next to your documents, built directly on @agentclientprotocol/sdk (protocol v1: fs read/write, terminals, permissions, session config options, slash commands, session resume).
• Agents install in one click from the ACP Registry — the app consumes cdn.agentclientprotocol.com/registry/v1/latest/registry.json live, including auth-method handling and managed runtimes for npx/uvx distributions. Registry additions show up for Open Knowledge users with no app update.
• ACP functionality of OpenKnowledge is built into the product itself and maintained by the core team.

Website: https://openknowledge.ai

[amikofalvy]

https://openknowledge.ai/blog/native-integrations-with-any-agent-via-acp