feat: add Claude Opus 4.8 model support

[Hansen1018]

Summary

• Problem: Add support for the new Claude Opus 4.8 model tier
• What changed: Updated default Claude model from opus-4-6 to opus-4-8 across all configs, added opus-4-8 to CLAW402_MODELS, BLOCKRUN_MODELS, pricing, and payment endpoints at $0.18/call
• What did NOT change (scope boundary): All other AI models and providers remain unchanged. opus-4-6 is retained as fallback option.

Change Type

• Feature

Scope

• MCP / AI clients
• API / server
• Web UI / frontend

Testing

• go build ./... passes
• npm run build passes (web)
• go test ./... passes

Compatibility

• Backward compatible? (Yes - opus-4-6 still available)
• Migration needed? (No)

[github-actions]

🤖 Advisory Check Results

These are advisory checks to help improve code quality. They won't block your PR from being merged.

📋 PR Information

Title Format: ✅ Good - Follows Conventional Commits
PR Size: 🟢 Small (20 lines: +14 -6)

🔧 Backend Checks

Go Formatting: ⚠️ Needs formatting

Files needing formatting

agent/execution_state.go
agent/model_provider_catalog.go
agent/skill_dag_runtime.go
agent/stock.go
api/errors.go
api/handler_wallet.go
cmd/e2e_builder_fee/main.go
kernel/formatter.go
kernel/grid_engine.go
kernel/validate_test.go

Go Vet: ✅ Good
Tests: ✅ Passed

Fix locally:

go fmt ./...      # Format code
go vet ./...      # Check for issues
go test ./...     # Run tests

⚛️ Frontend Checks

Build & Type Check: ✅ Success

Fix locally:

cd web
npm run build  # Test build (includes type checking)

---

📖 Resources

• Contributing Guidelines
• Migration Guide

Questions? Feel free to ask in the comments! 🙏

---

These checks are advisory and won't block your PR from being merged. This comment is automatically generated from pr-checks-run.yml.