Enhance default bot behavior with natural language routing

[google-labs-jules]

This PR enhances the bot's default behavior when no command is specified.

1. Routing Logic: Instead of always defaulting to GetBiblePassage, the bot now analyzes the message using ProcessNaturalLanguage:

   • If the message is a Bible Reference (e.g., "John 3:16"), it retrieves the passage.
   • If the message contains Extracted References (e.g., "Explain John 3:16"), it treats it as a query (GetBibleAsk) and passes the references as context.
   • If the message is a Short Phrase (< 5 words, no '?'), it performs a Keyword Search (GetBibleSearch).
   • Otherwise, it treats it as a question (GetBibleAsk).

2. New Command: Added /passage command to explicitly access the old default behavior (getting a passage).

3. Refactoring:

   • pkg/app/bible_reference.go was refactored to support partial reference matching (ParseBibleReferenceFromStart) and extraction (ExtractBibleReferences).
   • pkg/app/ask.go was updated to allow passing contextVerses to the API.

4. Testing:

   • Added TestExtractBibleReferences.
   • Added pkg/app/natural_language_test.go with 12+ test cases covering various routing scenarios including single chapter books, command-like phrases, and mixed inputs.

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PR created automatically by Jules for task 4998359254942843604 started by @julwrites

[google-labs-jules]

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