CLI extension architecture allows partial integrations without session support

[danshapiro]

Summary

CLI extensions can be added with a manifest, but session support still depends on handwritten provider code and a hardcoded registration list. That makes it easy to ship a provider that appears in the picker and can spawn, while sidebar discovery, session binding, history loading, and restart restore stay broken.

Current split

• server/extension-manifest.ts lets a CLI manifest define command, env vars, and resumeArgs.
• server/index.ts still builds codingCliProviders from [claudeProvider, codexProvider, opencodeProvider].
• server/coding-cli/providers/ contains handwritten providers for Claude, Codex, and OpenCode only.
• The CLI extensions refactor plan explicitly left server/coding-cli/ unchanged.

Why this is a problem

Kimi shows the failure mode:

• It has an extension manifest and terminal mode support.
• It does not have a session provider implementation or registration.
• Its manifest also omits resumeArgs.
• It can launch, but it does not appear in the sidebar from indexed sessions and does not restore cleanly after a server reset.

This is a product gap, but it is also an architectural one. The manifest system does not model session discovery or require a provider to exist before a CLI is treated as supported.

Requested change

Pick one direction and make it enforceable:

1. Extend the extension system so a CLI extension can define session discovery, parsing, project resolution, and history loading, then derive runtime registration from that data.
2. Keep handwritten providers, but make support status explicit:
   • A CLI should not be presented as supported unless both the spawn manifest and the session provider are registered.
   • Startup should validate that any built-in CLI extension with session claims has a matching provider.
   • Tests should cover picker visibility, sidebar indexing, session association, and restart restore for each supported CLI.

Acceptance criteria

• No CLI can appear supported with only spawn wiring.
• The provider registration list is not a second hidden source of truth.
• Adding a built-in CLI without session integration fails fast in tests or startup validation.
• Sidebar discovery and restart restore are covered for every supported CLI.