acp: agent registry manifest for IDE discovery

[bug-ops]

Parent: #762

Task

Create the agent manifest file that IDE clients use to discover and install Zeph as an ACP agent.

Acceptance criteria

• Agent manifest JSON with: name, version, description, transport (stdio), command, args, capabilities
• Publish-ready metadata matching ACP registry format
• README section on IDE integration (Zed, JetBrains, Neovim setup instructions)
• Verify agent appears correctly in at least one IDE's agent picker (manual test)