deps: replace SDK-backed GraphQL transport with native fetch

[iamfj]

What problem does this solve?

src/client/graphql-client.ts imports LinearClient from @linear/sdk only to call linearClient.client.rawRequest(...). This means Linearis cannot remove the SDK even though the transport can be implemented with Node 22 built-in fetch.

Proposed solution

Replace the SDK-backed GraphQL transport with a native implementation in src/client/graphql-client.ts.

The new transport should preserve existing behavior:

• POST to https://api.linear.app/graphql
• send JSON body { "query": print(document), "variables": variables }
• set Content-Type: application/json
• set Authorization consistently with current API token usage
• preserve public-file-urls-expire-in: 3600
• preserve request timeout behavior
• preserve withRetry() usage
• preserve current authentication/error mapping behavior

If print() remains in runtime code, add graphql as a direct pinned production dependency.

Alternatives considered

• Keep using LinearClient.client.rawRequest(...): simplest short term, but leaves the SDK dependency in the critical path.
• Add another GraphQL HTTP client dependency: unnecessary on Node 22 because fetch is available.

Primary use case

Shell scripting / automation

Additional context

Current files involved:

• src/client/graphql-client.ts
• tests/unit/client/graphql-client.test.ts
• package.json
• package-lock.json

Acceptance criteria:

• src/client/graphql-client.ts no longer imports @linear/sdk.
• Unit tests mock native fetch instead of the SDK.
• Existing timeout, retry, auth error, and GraphQL error behavior is covered.
• npm test, npm run check:ci, and typecheck pass.

[linear]

LIN-147