Attach a source document as a block reference under one or more configured
attach actions defined in `.iwe/config.toml`.

For each --to <NAME>:
1. Look up <NAME> in [actions]; the action must be of type `attach`.
2. Render the action's key_template to compute the target key
   (e.g. `daily/{{today}}` becomes `daily/2026-04-25`).
3. If the target document doesn't exist yet, create it with the action's
   `title` as the H1 and the new block reference as the body.
4. If the target exists, append the new block reference.
5. If the source is already attached in the target, that target is silently
   skipped — no error, no warning, no duplicate write.

The reference text on the new block is the source document's title.
