v0.1.0-rc7 Client builder API updated

⚠️ BREAKING CHANGES
- client: Builders are now created via client.NewX() instead
  of lunatask.NewX(). Create/Update methods no longer require
  a client parameter directly.
  Migration:
    Before: lunatask.NewTask("x").Create(ctx, client)
    After: client.NewTask("x").Create(ctx)

♻️ Code Refactoring
- (client): Moved builders to Client methods for better client
  ownership and API ergonomics.

🔧 Build System
- chore: Removed exhaustruct linter.
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