agent-sdk-ts parity: Plugins + custom tool loading (plugin model + directory loader + remote support) (deferred)

[enyst]

agent-sdk-ts is basically a transpilation of the original python SDK that we can find at enyst/agent-sdk (it’s a fork for our use). Clone yourself agent-sdk so you have the original codebase and you can understand the code deeply before transpiling.

Gap

Parity doc lists Python plugin/custom tool support missing in TS:

• plugin data model
• load tools from directories
• remote custom tools support

TS currently expects tools to be provided programmatically and RemoteConversation sends only a tool name list.

Python reference

• openhands-sdk/openhands/sdk/plugin/*
• tool loading in openhands-sdk/openhands/sdk/agent/* / tool/*

TS locations

• packages/agent-sdk-ts/src/tools/*
• packages/agent-sdk-ts/src/sdk/runtime/Agent.ts (tool registration)
• packages/agent-sdk-ts/src/sdk/conversation/RemoteConversation.ts (tool list sent to agent-server)

Implementation plan

1. Define a TS plugin model (JSON schema) matching the minimum Python plugin metadata.
2. Implement plugin discovery:
   • load from a configured directory (workspace + ~/.openhands)
   • validate schemas and tool definitions
3. Add custom tool loader:
   • map plugin tool definitions → ToolDefinition instances
4. Remote support:
   • include custom tool schemas when starting RemoteConversation (if agent-server supports it)
   • or provide a compatibility layer that uploads plugin manifests first.
5. Add tests with fixture plugin directories.

Acceptance criteria

• Users can drop a plugin/tool into a directory and have it become available without code changes.
• RemoteConversation can optionally include custom tools (or a clear error if server can’t).

[enyst]

### Dependency notes (stacking / merge order)
**Blocked by:** #646, #647, #661

**Rationale / why it’s stacked this way:**
- #646: Plugins/custom tools benefit from hook interception points (pre/post tool) for safety + wrapping.

• agent-sdk-ts parity: includeDefaultTools option (Agent tool selection parity) #647: Custom tools should integrate with tool selection/default-tool wiring rather than bypassing it.

• agent-sdk-ts parity: LLM registry/resolver + router + exception mapping parity #661: Error/exception normalization should be stable so plugin tool failures surface consistently.

  Likely blocks: (none)