add `TODO.md` to make it easier to revisit the project in spurts

TODO.md

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++ Break-out functionality in `sendMessage` into separate procedures,
+  namespaced by domain (e.g. `messages`, `facts`, `factTriggers`).
+  + Make a `withDbWrite` function to wrap the calls to these procedures, so
+    as to separate the generation of data from persisting it.
++ Somehow subscribe the UI to events from the `sendMessage` procedure,
+  so the user knows what's going on.
++ Parallelize the generation of data, so that the UI doesn't freeze for
+  so long while the data is being generated.
++ Chunk and generate embeddings for generated facts and fact triggers.
+  And save them to a database.
++ Query the database for facts triggers that match the current
+  conversation situation, and inject the facts they refer to into the
+  conversation context.
++ Implement tool use. Including tool triggers similar to fact triggers.