fix(core): normalize pi-sdk azure base url

[christso]

Summary

• normalize bare Azure resource names to OpenAI-compatible HTTPS /openai/v1 base URLs for the pi-coding-agent SDK path
• apply the normalized URL both to SDK env wiring and to the model object passed into createAgentSession
• add focused tests for the Azure normalization helper and the PiCodingAgentProvider env mapping
• clarify AGENTS.md so future feature work starts from a fresh worktree based on latest origin/main

Red

• On origin/main in this worktree, a real Azure-backed repro failed with:
  • pi-coding-agent SDK error (provider: openai, model: gpt-5.4-mini): "leos-m6pmw8kz-eastus2/responses" cannot be parsed as a URL.
• Command:
  • GRADER_TARGET=azure bun apps/cli/src/cli.ts eval run examples/features/deterministic-evaluators/evals/dataset.eval.yaml --test-id equals-exact --target pi-sdk-azure
• Focused red test:
  • bun test packages/core/test/evaluation/providers/pi-coding-agent.test.ts

Green

• Focused tests:
  • bun test packages/core/test/evaluation/providers/pi-coding-agent.test.ts packages/core/test/evaluation/providers/pi-provider-aliases.test.ts
• Real eval:
  • GRADER_TARGET=azure bun apps/cli/src/cli.ts eval run examples/features/deterministic-evaluators/evals/dataset.eval.yaml --test-id equals-exact --target pi-sdk-azure
  • Result: 1/1 passed, mean 1.000
• Repo checks:
  • bun run build
  • bun run test
  • bun run lint

Closes #931

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