chore(backlog): decompose Gap 12.1 into sub-tasks

[julwrites]

Decomposed Gap 12.1 in BACKLOG.md into two separate sub-tasks (embed and embed-multi) as the original task was assessed to modify more than 3 files (lua/llm/commands.lua, lua/llm/facade.lua, tests/spec/commands_spec.lua, tests/spec/facade_spec.lua). Preserved ranking and updated numbering.

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PR created automatically by Jules for task 13329055908318333585 started by @julwrites

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