Fix clientId usage to follow LiveStore best practices

[rgbkrk]

This PR fixes the incorrect usage of clientId in the runt runtime agent system.

Problem

Previously, the system was setting clientId = userId, which goes against LiveStore's design principles. According to the LiveStore docs and guidance from the maintainer @schickling:

• clientId should identify device/app instances, not users
• Multiple devices/tabs from the same user should have different clientIds
• User identification should be handled separately in the sync payload

Solution

1. Modified discoverUserIdentity() to return both user info and generate a proper clientId using the pattern userId-{uniqueId}

2. Updated authentication flow to:

   • Generate unique clientId for each runtime instance
   • Pass userId separately in sync payload for authorization
   • Maintain backward compatibility with existing auth patterns

3. Updated schema documentation to reflect correct LiveStore patterns

4. Fixed all tests to include the required userId parameter

Changes

• packages/pyodide-runtime-agent/src/auth.ts: Enhanced to return both userId and generated clientId
• packages/lib/src/types.ts: Added userId to RuntimeAgentOptions
• packages/lib/src/config.ts: Added userId to RuntimeConfig
• packages/lib/src/runtime-agent.ts: Added userId to sync payload
• packages/schema/mod.ts: Updated authentication pattern documentation
• All test files: Added userId parameter to RuntimeConfig instances