fix: send asknews-cli/<version> as User-Agent

[wagnercosta]

🔴 Review tier: HIGH — human review required

• Release: a fix: on main feeds the next release-please Release PR, which publishes to npm

What

Every request the CLI sends to the API gateway now carries User-Agent: asknews-cli/<package version>, next to the SDKs' own asknews-sdk-<lang> agents. The gateway uses it to attribute usage to the CLI channel; until now CLI traffic was indistinguishable from raw API calls.

• src/lib/version.ts: single source of the package version and the user agent string (cli.ts reads the version from it too).
• src/lib/http.ts: header set in both request builders (raw requests and schema operations, buffered and streamed).
• No change to arguments, output, or credentials handling.

Verified

• pnpm lint and pnpm typecheck pass.
• pnpm test: the 4 new tests in test/user-agent.test.ts pass (raw, operation and streamed requests all send the header). One failure in test/output.test.ts (web search table rendering) is pre-existing on main and unrelated.

Umbrella: em-hub/features/usage-client-id/