fix(milestones): use $input convention for milestone mutations

[iamfj]

What does this PR do?

linearis milestones create failed at runtime with a GraphQL variable error and milestones update sent incorrect variables, because project-milestones.graphql was the only mutation file declaring top-level scalar variables while the service passed them under the $input convention used everywhere else. This aligns both CreateProjectMilestone and UpdateProjectMilestone to take a single $input: ...Input! argument and updates the service to pass { input } / { id, input } accordingly.

Closes #223

Type of change

• Bug fix
• New feature
• Refactor (no behavior change)
• Documentation
• Tests
• Build / CI

Checklist

• npm run check:ci passes (lint + format)
• npx tsc --noEmit passes (type check)
• npm test passes (unit tests)
• New code has tests (happy path + primary error case)
• Commit messages follow Conventional Commits

Testing

npx tsc --noEmit is clean and npx vitest run tests/unit/services/milestone-service.test.ts passes (12 tests), including two new assertions that create/update forward inputs as single GraphQL variables ({ input } and { id, input }).

Notes for reviewers

src/gql/ is regenerated via npm run generate (gitignored) so the change is type-correct end-to-end.