Format graphiti edge data into the canonical fact strings the LLM sees.

Pure Elixir — graphiti runs in Python, but we extract its edges into Elixir maps and format them here, never reaching back into Python for formatting.

Output shape per fact:

- {fact} (created …) (valid from …) (invalid since …) (expired …)

Timestamp normalisation matches the server's pipelines/formatting.py so consumers see identical fact text from either stack.

See ex-format-fact in gralkor/TEST_TREES.md.