fix(agents): rename app_api.agents package to unbreak local app-api startup

[philmerrell]

Local app-api startup crashes: ModuleNotFoundError: No module named 'agents.main_agent'

Root cause

run-app-api.sh launches app-api with cd src/apis/app_api && python main.py, so src/apis/app_api lands on sys.path[0]. The apis/app_api/agents/ package added for the Agent Designer (#591/#592) then shadows the top-level agents package — admin/quota/routes.py's from agents.main_agent.quota.repository import ... resolves into it, which has no main_agent, and startup dies.

• Tests didn't catch it: pytest runs from backend/ (pythonpath=src), where import agents correctly resolves to backend/src/agents. The collision only appears when the CWD is the app_api dir.
• Production is NOT affected: the container runs uvicorn apis.app_api.main:app from WORKDIR /app, so sys.path[0] is /app and import agents finds the top-level package. Only local dev via run-app-api.sh (the CLAUDE.md-documented command) broke.

Fix

Rename apis/app_api/agents/ → apis/app_api/agent_designer/ (and the mirror test dir) so the package name can't collide with the top-level agents package. Pure rename — the /agents URL surface, the agents_router, and all behavior are unchanged; only the internal Python module path moves.

Verification

• Reproduced the original failure from src/apis/app_api, then confirmed the fix: import agents now resolves to src/agents, import agents.main_agent.quota.repository succeeds, and the full apis.app_api.main module loads from that directory with the /agents routes still mounted.
• 34 tests pass (agent routes + binding validation + architecture import-boundary gate).

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