fix: prevent Stage 4 literature search from silently falling back

[pikaqiu2333]

Summary

• Remove the function-local os import that broke environment-variable lookup in Stage 4.
• Restore Semantic Scholar and OpenAlex API key resolution from configuration and environment variables.
• Add regression coverage for key precedence, search configuration, Web Search augmentation, orchestrator execution, and fallback behavior.

Root Cause

A function-local import os caused Python to treat os as a local variable throughout _execute_literature_collect.

Stage 4 accessed os.environ before reaching that import, raising UnboundLocalError. The existing broad exception handler then caught the error and silently skipped the real literature APIs.

This regression was introduced by my changes in #299. I’m sorry I missed the interaction with the existing function-local import during testing. Thank you for reporting it in #309. This PR fixes the issue and adds orchestration-level regression coverage to prevent the same silent fallback from recurring.

Validation

• Stage 4 regression tests: 8 passed
• Configuration and literature tests: 78 passed
• Executor tests: 207 passed
• Full suite: 2869 passed, 57 skipped
• Real literature retrieval:
  • 18 real candidates
  • 12 OpenAlex and 6 arXiv results
  • 0 placeholders
  • 18 BibTeX entries
  • Semantic Scholar rate limiting was handled without breaking the remaining providers

Two pre-existing Windows shell-related tests remain environment-specific and are unrelated to this change.

Fixes #309