ci: put npm's global bin on PATH for the dx CLI

[wittjosiah]

npm published. @dxos/plugin-tictactoe@0.10.1 is live — dist-tags.latest moved from the 0.0.0 placeholder to 0.10.1 — and tagged. The repository.url fix worked; that half of the pipeline is proven end to end.

The registry publish still didn't run (run 31267297719):

added 1 package in 6s
/home/runner/work/_temp/….sh: line 3: dx: command not found
##[error]Process completed with exit code 127

The install succeeded and the binary was still unresolvable, because node comes from proto and npm's global bin is not a directory the shell already searches.

Change

export PATH="$(npm prefix -g)/bin:${PATH}"
npm install -g "${DX_CLI_PACKAGE}"
dx --version

The same npm answers prefix -g and performs the install, so the location is correct by construction — no second guess about where the binary went, and nothing invented for it to live in.

Verification

Uninstalled the CLI, confirmed dx was genuinely unresolvable, then ran the step's lines extracted from the workflow YAML so the test can't drift from what ships, with proto's node first on PATH as on the runner:

dx before                → none
export PATH=…            → /opt/node22/bin
npm install -g …         → added 1 package in 7s
dx --version             → 0.10.0, exit 0

(An earlier attempt of mine reported success against a stale CLI still on PATH from a previous test; the run above is after removing it.)

Next

npm is append-only, so a plain re-run finds 0.10.1 already published, reports nothing released, and skips the registry step — the trap registry_only was added for in #26. Once this lands I'll dispatch Release with registry_only: true to drive the registry half, with no version burn per attempt.