Refactor agent-service

[bobbai00]

Task Summary

Umbrella for refactoring the agent-service codebase into clean, single-topic changes — no runtime behavior change. The work is split into small, independently reviewable PRs that build and pass tests on their own, landed in dependency order. Each sub-task is tracked as a sub-issue below.

Sub-tasks

• refactor(agent-service): consolidate the WebSocket message-type framework #5749 — consolidate the WebSocket message-type framework → refactor(agent-service): extract WebSocket message types into types/ws #5751
• refactor(agent-service): remove the dead checkout/headChange path #5942 — remove the dead checkout/headChange path → refactor(agent-service): remove dead checkout/headChange path #5930
• refactor(agent-service): centralize backend DTOs, metadata, endpoint config, and auth #5748 — centralize backend DTOs, metadata, endpoint config, and auth → refactor(agent-service): centralize backend DTOs, metadata, endpoint config, and auth #5928
• refactor(agent-service): redesign the sync-execution result and error model #5750 — redesign the sync-execution result and error model → refactor(agent-service): redesign sync-execution result and error model #5927

Supersedes the earlier all-in-one refactor (#5564) and the broad cleanup (#5563); agent-service test-coverage (#5266) is folded into each sub-task.

Task Type

• Refactor / Cleanup
• DevOps / Deployment / CI
• Testing / QA
• Documentation
• Performance
• Other