doc: update multiprocess docs for BGL names

[pinehill99]

Summary:

• Update multiprocess usage and design docs to use the Bitgesell executable names: BGLd, BGL-qt, BGL-node, BGL-wallet, and BGL-gui.
• Align the libraries design doc with the repository's libBGL_* and BGLconsensus naming, including the dependency diagram.
• Keep upstream Bitcoin links only where they point to historical design references.

Validation:

• rg -n 'bitcoin-node|bitcoin-wallet|bitcoin-gui|bitcoin-cli|bitcoind|bitcoin-qt|libbitcoin|Bitcoin Core|Multiprocess Bitcoin|BITCOIN_SOURCE_DIRECTORY' doc/design/libraries.md doc/design/multiprocess.md doc/multiprocess.md
• python3 test/lint/lint-tests.py
• git diff --check HEAD~1..HEAD

Bounty:

• Refs BGL PR bounty hunt ($10000 overall budget) #39