Supersedes #6178.
This branch keeps the exact same final content as #6178, but splits
history for reviewability:
1. `chore(companion): rename source and test files from .js to .ts`
(pure `git mv`, no content changes)
2. `feat(companion): port Companion to TypeScript` (actual
content/type/schema/build updates)
Validation note:
- Final tree is byte-for-byte identical to `ts-companion`
(`7b5b16a298690b0492de4cc4fab744f998b45570`).
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Co-authored-by: Mikael Finstad <finstaden@gmail.com>
