feat: add assistant mode, SSE streaming, and OpenCode server integration

[chriswritescode-dev]

Summary

• Add assistant mode with internal token authentication and API routes
• Add OpenCode server authentication settings and config
• Add SSE password resolver and server health monitoring
• Integrate opencode event stream with SSE aggregator
• Update README and project documentation
• Bump version to 0.10.0

Changes

• backend/src/services/assistant-mode.ts - New assistant mode service
• backend/src/services/opencode/client.ts - New OpenCode event stream client
• backend/src/services/opencode/auth.ts - New OpenCode auth service
• backend/src/services/internal-token.ts - Internal token authentication
• backend/src/routes/internal/* - New internal API routes (notifications, settings)
• backend/test/routes/internal-*.test.ts - New internal API tests
• backend/test/services/opencode/client.test.ts - New client tests
• frontend/src/hooks/useOpenCode.ts - New OpenCode hook
• frontend/src/hooks/useOpenCodeServerAuth.ts - New auth hook
• frontend/src/lib/opencode-event-stream/* - New event stream implementation
• frontend/src/components/settings/OpenCodeServerAuthSettings.tsx - New settings component
• frontend/src/components/repo/RepoSkillsDialog.tsx - New skills dialog
• shared/src/schemas/internal-assistant.ts - New assistant schemas
• docs/features/assistant-internal-api.md - New docs
• • many more (133 files changed, 7278 insertions, 6287 deletions)

Type of Change

• Feature
• Refactor

Checklist

• Code follows project style (no comments, named imports)
• TypeScript types are properly defined
• Tests added/updated
• `pnpm lint` passes locally