Three compounding bugs caused the consolidation LLM to replace
user-defined tags (e.g. 'bugfix', 'progress') with invented names
(e.g. 'Productivity Style'):

1. build_consolidation_prompt() never received valid tag names, so the
   LLM had no knowledge of which tags were allowed.
2. _features_to_llm_format() stripped metadata IDs before sending
   features to the consolidation LLM, making keep_memories unreliable.
3. _deduplicate_features() wrote LLM-returned tags directly to storage
   with no validation, allowing silent tag renaming.

Fixes: pass tags to consolidation prompt, add ID-preserving serializer
for consolidation, and enforce original tag on consolidated features.