fix: clarify docs import LLM configuration hint

[deep231w]

closes #92
What this changes

Updates the docs import error hint to mention only llm.api_key and PIKOPOD_LLM_KEY. Adds a test to verify the message.

Why

Users need clear, provider-independent instructions for configuring the LLM when importing documentation without a machine-readable spec.

Checklist

• go test ./... -count=1 passes
• gofmt -l cmd internal is empty and go vet ./... is clean
• No new cgo dependency (the release artifact is a static binary on every platform)
• Error message strings are unchanged, or the change is deliberate and called out above
• New test fixtures have an entry in testdata/parity/importer/specs/SOURCES.md
• No provider payloads, keys, tokens, or customer data in the diff or in test fixtures