[Bug]: Process-global subagent registry leaks definitions across conversations

Bug Description

File/plugin subagent definitions are conversation-specific, but every factory is stored in one process-global first-writer-wins registry (openhands-sdk/openhands/sdk/subagent/registry.py:56-58,124-152,288-367).

Each LocalConversation registers its plugin and workspace definitions into that registry (conversation/impl/local_conversation.py:1211-1216,1482-1499). Two conversations can therefore define the same subagent name differently; whichever initializes first silently controls both. Remote creation also serializes all definitions visible in the process (remote_conversation.py:807-833), which can include definitions from unrelated workspaces. Agent Server repeats the same global registration for per-conversation payloads (conversation_service.py:513-545).

Expected Behavior

Built-in/programmatic definitions may have process ownership, but plugin/project/user file definitions must resolve in the conversation that discovered them. Same-name definitions in different conversations must not collide or leak.

Actual Behavior

An independent, self-contained reproducer is posted in the issue comments. Save
it as /tmp/repro_4661.py, then run:

uv run python /tmp/repro_4661.py

On SDK commit 6d3881035, it exercises two real project trees and the actual file-agent registry; the test-only reset is used solely to run both initialization orders independently and prints:

first_project=project-a
first_registered=['shared-repro-agent']
second_registered=[]
global_winner=definition-from-A
remote_forwarded_definition=['definition-from-A']
first_project=project-b
first_registered=['shared-repro-agent']
second_registered=[]
global_winner=definition-from-B
remote_forwarded_definition=['definition-from-B']

Suggested Implementation

Retain a process-level base registry for explicit programmatic/built-in registrations and add a conversation-scoped overlay/resolver for plugin/project/user definitions. Task/delegate lookup and remote serialization should consume that resolver.

Acceptance Criteria

• Two live local conversations can use same-name, different project definitions in either initialization order.
• Two server conversations can do the same concurrently.
• Remote creation serializes only definitions available to that conversation.
• Existing precedence remains: programmatic > plugin > project > user > built-in.
• Persisted/remote resume and task/delegate lookup remain compatible.

Version

Current main at 6d3881035. Found during the independent coverage pass of a read-only ownership audit.

Start with the reproducer at /tmp/repro_4661.py, then trace registry.py, local_conversation.py, remote_conversation.py, and conversation_service.py at the cited ranges. Follow how definitions are registered, resolved, serialized, resumed, and used for task/delegate lookup. Done means the listed acceptance checks pass for local, server, remote, precedence, and resume behavior.