Give the browser extension a real package.json and CI wiring

[JSONbored]

Problem: apps/gittensory-miner-extension isn't an npm workspace member and has zero references anywhere in CI — no lint, no typecheck, no build step of its own, unlike its sibling gittensory-extension, which is wired in.

Area: AMS / Testing

Proposal: Give the extension a real package.json, add it as a workspace member, and wire it into CI (lint/typecheck/build) matching its sibling.

Deliverables:

• A real workspace package with CI coverage matching gittensory-extension's.

[linear-code]

JSO-3526