Epic: Delete Fix Agent backend code

[hiydavid]

Context

Follow-up to #179 (Level Up (May)), which removes the Fix Agent UI but leaves backend code in place as dead code. This epic tracks the full backend deletion once the UI removal has baked for a release and no regressions surface.

Goal

Delete the now-unused Fix Agent backend code, routes, prompts, and tests.

Prerequisites

• Epic: Consolidate Fix-Path into Auto-Optimize #179 shipped and deployed
• At least one full release cycle has passed with no regressions or rollback requests
• No external consumers of POST /api/spaces/{id}/fix identified (the frontend stopped calling it; confirm via access logs or a search through external integrations)

Work

• Delete backend/services/fix_agent.py
• Remove get_fix_agent_single_prompt() from backend/prompts.py
• Remove FixRequest from backend/models.py
• Remove POST /api/spaces/{id}/fix endpoint from backend/routers/spaces.py
• Delete backend/tests/test_fix_agent_prompt.py
• Delete backend/tests/test_fix_agent_paths.py
• Update docs/appendices/A-api-reference.md — remove the deprecated endpoint row entirely
• Delete docs/06-fix-agent.md if kept-with-banner during Epic: Consolidate Fix-Path into Auto-Optimize #179
• Remove any remaining CLAUDE.md references to the Fix Agent

Acceptance

• No reference to fix_agent, FixAgent, FixRequest, or get_fix_agent_single_prompt remains anywhere in the codebase
• pytest backend/tests/ passes
• Deployed app works normally; no 404/500 regressions from removed endpoint
• Docs have no broken links

Files (to delete or modify)

• backend/services/fix_agent.py (delete)
• backend/prompts.py
• backend/models.py
• backend/routers/spaces.py
• backend/tests/test_fix_agent_prompt.py (delete)
• backend/tests/test_fix_agent_paths.py (delete)
• docs/06-fix-agent.md (delete if still present)
• docs/appendices/A-api-reference.md
• CLAUDE.md