CLI's `Sibit::Bin#client` never passes a key to `Sibit::Blockchair.new` at `lib/sibit/bin.rb:159`

`Sibit::Blockchair.new` accepts a `key:` keyword argument that the constructor stores at `lib/sibit/blockchair.rb:22-26` and that `balance`, `push`, and the rest of the adapter append to the upstream URL when it is set. The CLI builds Blockchair at `lib/sibit/bin.rb:159` with `Sibit::Blockchair.new(http: http, log: log)` and never reads the key from any source, so a user who runs `sibit --api=blockchair balance ...` against an authenticated Blockchair endpoint can only hit the unauthenticated tier and gets the lower rate limit.

The same file already wires `Sibit::Cryptoapis` to an environment variable at `lib/sibit/bin.rb:164`: `Sibit::Cryptoapis.new(ENV.fetch('SIBIT_CRYPTOAPIS_KEY', nil), http: http, log: log)`. Blockchair has no equivalent, so the `key:` parameter added together with the URL-fragment fix in issue #256 stays inaccessible from the command line.

Read the key from a `SIBIT_BLOCKCHAIR_KEY` environment variable (mirroring the `SIBIT_CRYPTOAPIS_KEY` convention) and forward it: `Sibit::Blockchair.new(key: ENV.fetch('SIBIT_BLOCKCHAIR_KEY', nil), http: http, log: log)`. A short README note next to the existing `SIBIT_CRYPTOAPIS_KEY` mention would document the new variable.