refactor(config): consider moving [llm.extraction] under [extraction.llm]

[cpcloud]

Context

Currently the LLM extraction config lives under [llm.extraction] as a parallel override to [llm.chat]. Both inherit from [llm].

With the addition of [extraction.ocr] in #729, all extraction-related config would read more naturally grouped under [extraction]:

[extraction]
enable = true

[extraction.llm]       # currently [llm.extraction]
provider = "anthropic"
model = "claude-haiku-3-5-20241022"

[extraction.ocr]
enable = true
confidence_threshold = 70

Trade-offs

• Pro: All extraction config in one subtable, easier to find
• Con: Breaks the current symmetry where [llm] is the base and [llm.chat]/[llm.extraction] are parallel overrides
• Con: [llm.chat] would be the only pipeline override left under [llm], making the inheritance pattern asymmetric

Migration

Would need TOML key + env var deprecation entries for the old paths, following the same pattern as other renames in migrateRenamedKeys / envRenames.