feat(linear): add Issue.priorityLabel

[ikatkov]

Summary

Closes #219.

• Add Linear's non-null Issue.priorityLabel field to the emulated GraphQL schema.
• Derive the label in the shared issue formatter for queries, nested issue objects, and mutation payloads.
• Cover all five documented priority tiers, omitted priorities, fractional values, and invalid stored values.
• Document the supported field across the CLI help and Linear documentation.

Reproduction

On emulate@0.10.0, this valid Linear-shaped query is rejected during document validation:

query {
  issues(first: 1) {
    nodes {
      id
      priority
      priorityLabel
    }
  }
}

The response is HTTP 400 with Cannot query field "priorityLabel" on type "Issue", so the same shared issue fragment also blocks mutations such as issueCreate before they execute.

With this change, issue reads return the documented mapping:

priority	priorityLabel
0	No priority
1	Urgent
2	High
3	Medium
4	Low

An issueCreate mutation with no priority now succeeds when selecting priorityLabel and returns { priority: 0, priorityLabel: "No priority" }.

Verification

• pnpm --filter @emulators/linear test
• pnpm --filter @emulators/linear type-check
• pnpm --filter @emulators/linear lint
• pnpm --filter @emulators/linear build
• pnpm --filter emulate lint
• Prettier and git diff --check

Separate schema discrepancy

Linear's published schema types Issue.priority as Float!, while the emulator currently declares Int!. This PR intentionally leaves that potentially breaking type change out of the additive priorityLabel fix.

[vercel]

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