fix: pass providerConfig envVars to startPanel on session creation

[everpcpc]

Summary

Provider ENV settings configured in Settings (providerConfigs.{claude,codex,gemini,kimi}.envVars) were not applied when a session was first created. They only took effect on subsequent messages (the panels:continue path).

The root cause was that TaskQueue.ts did not call readProviderConfig() when invoking startPanel() for any of the four providers, while session.ts correctly did so in the panels:continue handler.

This fix:

• Exports readProviderConfig from session.ts
• Imports it in TaskQueue.ts and passes providerConfig to all four startPanel calls (Claude, Codex, Gemini, Kimi)

Tests

• Unit Test
• Logic Test
• Benchmark Test
• No Test - Two-line plumbing fix passing an existing config to existing callsites; covered by manual verification

Type of change

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that could cause existing functionality not to work as expected)
• Documentation Update
• Refactoring
• Performance Improvement
• Other (please describe):