Backend resolution now defers until after file detection. A code-only
corpus (pure AST, zero LLM calls) runs without any API key.
Key validation only fires when needs_llm=True (semantic_files non-empty
or --dedup-llm passed). Error message now names why a key is needed and
notes that code-only corpora need none.
Applied from PR #1123 with one fix: _clear_backend_keys in tests now
also clears AWS_PROFILE/REGION, OLLAMA_BASE_URL to prevent CI flakes
on machines with ambient credentials.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Safi
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