* fix(retain): preserve normalized experience fact types and remove deprecated opinion type
The ExtractedFactType conversion was re-checking for raw "assistant" fact_type
after the parsing layer had already normalized it to "experience". Since
fact_from_llm.fact_type was always "experience" (never "assistant"), the ternary
always fell through to "world", silently losing experience classification.
Also removes the deprecated "opinion" fact type from internal extraction models,
database constraints/indexes (via migration), and dead code paths. The public API
surface (descriptions, response models, backwards-compat filter) is unchanged.
* refactor(retain): drop unused confidence_score column
The confidence_score column was only ever non-null for opinion facts
(which are now removed). It was always written as NULL and never read
back from the database. Remove it from:
- DB model and migration (DROP COLUMN)
- INSERT queries in fact_storage.py
- retain_async/retain_batch_async parameters
- RetainContext/RetainResult extension models
- RetainBatch dataclass
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